diff --git a/Doc/library/sys.rst b/Doc/library/sys.rst
index 24ce207..a98c5d4 100644
--- a/Doc/library/sys.rst
+++ b/Doc/library/sys.rst
@@ -507,6 +507,17 @@
    when Python is started with the -3 option.
 
 
+.. data:: dont_write_bytecode
+
+   If this is true, Python won't try to write ``.pyc`` or ``.pyo`` files on the
+   import of source modules.  This value is initially set to ``True`` or ``False``
+   depending on the ``-B`` command line option and the ``PYTHONDONTWRITEBYTECODE``
+   environment variable, but you can set it yourself to control bytecode file
+   generation.
+
+   .. versionadded:: 2.6
+
+
 .. function:: setcheckinterval(interval)
 
    Set the interpreter's "check interval".  This integer value determines how often
diff --git a/Doc/using/cmdline.rst b/Doc/using/cmdline.rst
index 5f6bda5..b113c8f 100644
--- a/Doc/using/cmdline.rst
+++ b/Doc/using/cmdline.rst
@@ -145,6 +145,14 @@
 Miscellaneous options
 ~~~~~~~~~~~~~~~~~~~~~
 
+.. cmdoption:: -B
+
+   If given, Python won't try to write ``.pyc`` or ``.pyo`` files on the
+   import of source modules.  See also :envvar:`PYTHONDONTWRITEBYTECODE`.
+
+   .. versionadded:: 2.6
+
+
 .. cmdoption:: -d
 
    Turn on parser debugging output (for wizards only, depending on compilation
@@ -411,3 +419,10 @@
    If this is set, Python ignores case in :keyword:`import` statements.  This
    only works on Windows.
 
+
+.. envvar:: PYTHONDONTWRITEBYTECODE
+
+   If given, Python won't try to write ``.pyc`` or ``.pyo`` files on the
+   import of source modules.
+
+   .. versionadded:: 2.6
diff --git a/Include/pydebug.h b/Include/pydebug.h
index 3282dc1..1d24853 100644
--- a/Include/pydebug.h
+++ b/Include/pydebug.h
@@ -17,6 +17,7 @@
 PyAPI_DATA(int) Py_UnicodeFlag;
 PyAPI_DATA(int) Py_IgnoreEnvironmentFlag;
 PyAPI_DATA(int) Py_DivisionWarningFlag;
+PyAPI_DATA(int) Py_DontWriteBytecodeFlag;
 /* _XXX Py_QnewFlag should go away in 3.0.  It's true iff -Qnew is passed,
   on the command line, and is used in 2.2 by ceval.c to make all "/" divisions
   true divisions (which they will be in 3.0). */
diff --git a/Misc/NEWS b/Misc/NEWS
index 3896dd8..dbc0034 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -12,6 +12,10 @@
 Core and builtins
 -----------------
 
+- Patch #602345: Add -B command line option, PYTHONDONTWRITEBYTECODE envvar
+  and sys.dont_write_bytecode attribute. All these can be set to forbid Python
+  to attempt to write compiled bytecode files.
+
 - Improve some exception messages when Windows fails to load an extension
   module. Now we get for example '%1 is not a valid Win32 application' instead
   of 'error code 193'.
diff --git a/Modules/main.c b/Modules/main.c
index 26f8780..71c9c59 100644
--- a/Modules/main.c
+++ b/Modules/main.c
@@ -40,7 +40,7 @@
 static int  orig_argc;
 
 /* command line options */
-#define BASE_OPTS "3c:dEhim:OQ:StuUvVW:xX?"
+#define BASE_OPTS "3Bc:dEhim:OQ:StuUvVW:xX?"
 
 #ifndef RISCOS
 #define PROGRAM_OPTS BASE_OPTS
@@ -59,39 +59,42 @@
 /* Long usage message, split into parts < 512 bytes */
 static char *usage_1 = "\
 Options and arguments (and corresponding environment variables):\n\
+-B     : don't write .py[co] files on import; also PYTHONDONTWRITEBYTECODE=x\n\
 -c cmd : program passed in as string (terminates option list)\n\
 -d     : debug output from parser; also PYTHONDEBUG=x\n\
 -E     : ignore environment variables (such as PYTHONPATH)\n\
 -h     : print this help message and exit (also --help)\n\
 -i     : inspect interactively after running script; forces a prompt even\n\
-         if stdin does not appear to be a terminal; also PYTHONINSPECT=x\n\
 ";
 static char *usage_2 = "\
+         if stdin does not appear to be a terminal; also PYTHONINSPECT=x\n\
 -m mod : run library module as a script (terminates option list)\n\
 -O     : optimize generated bytecode slightly; also PYTHONOPTIMIZE=x\n\
 -OO    : remove doc-strings in addition to the -O optimizations\n\
 -Q arg : division options: -Qold (default), -Qwarn, -Qwarnall, -Qnew\n\
 -S     : don't imply 'import site' on initialization\n\
 -t     : issue warnings about inconsistent tab usage (-tt: issue errors)\n\
--u     : unbuffered binary stdout and stderr; also PYTHONUNBUFFERED=x\n\
 ";
 static char *usage_3 = "\
+-u     : unbuffered binary stdout and stderr; also PYTHONUNBUFFERED=x\n\
          see man page for details on internal buffering relating to '-u'\n\
 -v     : verbose (trace import statements); also PYTHONVERBOSE=x\n\
          can be supplied multiple times to increase verbosity\n\
 -V     : print the Python version number and exit (also --version)\n\
 -W arg : warning control; arg is action:message:category:module:lineno\n\
 -x     : skip first line of source, allowing use of non-Unix forms of #!cmd\n\
+";
+static char *usage_4 = "\
 -3     : warn about Python 3.x incompatibilities\n\
 file   : program read from script file\n\
 -      : program read from stdin (default; interactive mode if a tty)\n\
-";
-static char *usage_4 = "\
 arg ...: arguments passed to program in sys.argv[1:]\n\n\
 Other environment variables:\n\
 PYTHONSTARTUP: file executed on interactive startup (no default)\n\
 PYTHONPATH   : '%c'-separated list of directories prefixed to the\n\
                default module search path.  The result is sys.path.\n\
+";
+static char *usage_5 = "\
 PYTHONHOME   : alternate <prefix> directory (or <prefix>%c<exec_prefix>).\n\
                The default module search path uses %s.\n\
 PYTHONCASEOK : ignore case in 'import' statements (Windows).\n\
@@ -110,7 +113,8 @@
 		fprintf(f, usage_1);
 		fprintf(f, usage_2);
 		fprintf(f, usage_3);
-		fprintf(f, usage_4, DELIM, DELIM, PYTHONHOMEHELP);
+		fprintf(f, usage_4, DELIM);
+		fprintf(f, usage_5, DELIM, PYTHONHOMEHELP);
 	}
 #if defined(__VMS)
 	if (exitcode == 0) {
@@ -337,6 +341,10 @@
 			Py_OptimizeFlag++;
 			break;
 
+		case 'B':
+			Py_DontWriteBytecodeFlag++;
+			break;
+
 		case 'S':
 			Py_NoSiteFlag++;
 			break;
diff --git a/Python/import.c b/Python/import.c
index e191bd5..8094f7d 100644
--- a/Python/import.c
+++ b/Python/import.c
@@ -950,8 +950,11 @@
 		if (Py_VerboseFlag)
 			PySys_WriteStderr("import %s # from %s\n",
 				name, pathname);
-		if (cpathname)
-			write_compiled_module(co, cpathname, mtime);
+		if (cpathname) {
+			PyObject *ro = PySys_GetObject("dont_write_bytecode");
+			if (ro == NULL || !PyObject_IsTrue(ro))
+				write_compiled_module(co, cpathname, mtime);
+		}
 	}
 	m = PyImport_ExecCodeModuleEx(name, (PyObject *)co, pathname);
 	Py_DECREF(co);
diff --git a/Python/pythonrun.c b/Python/pythonrun.c
index adae679..4939f54 100644
--- a/Python/pythonrun.c
+++ b/Python/pythonrun.c
@@ -72,6 +72,7 @@
 int Py_InteractiveFlag; /* Needed by Py_FdIsInteractive() below */
 int Py_InspectFlag; /* Needed to determine whether to exit at SystemError */
 int Py_NoSiteFlag; /* Suppress 'import site' */
+int Py_DontWriteBytecodeFlag; /* Suppress writing bytecode files (*.py[co]) */
 int Py_UseClassExceptionsFlag = 1; /* Needed by bltinmodule.c: deprecated */
 int Py_FrozenFlag; /* Needed by getpath.c */
 int Py_UnicodeFlag = 0; /* Needed by compile.c */
@@ -172,6 +173,8 @@
 		Py_VerboseFlag = add_flag(Py_VerboseFlag, p);
 	if ((p = Py_GETENV("PYTHONOPTIMIZE")) && *p != '\0')
 		Py_OptimizeFlag = add_flag(Py_OptimizeFlag, p);
+	if ((p = Py_GETENV("PYTHONDONTWRITEBYTECODE")) && *p != '\0')
+		Py_DontWriteBytecodeFlag = add_flag(Py_DontWriteBytecodeFlag, p);
 
 	interp = PyInterpreterState_New();
 	if (interp == NULL)
diff --git a/Python/sysmodule.c b/Python/sysmodule.c
index 2a6a8c3..bd551b5 100644
--- a/Python/sysmodule.c
+++ b/Python/sysmodule.c
@@ -1128,6 +1128,9 @@
 	v = Py_BuildValue("(ssz)", "CPython", branch, svn_revision);
 	PyDict_SetItemString(sysdict, "subversion", v);
 	Py_XDECREF(v);
+        PyDict_SetItemString(sysdict, "dont_write_bytecode",
+                             v = PyBool_FromLong(Py_DontWriteBytecodeFlag));
+        Py_XDECREF(v);
 	/*
 	 * These release level checks are mutually exclusive and cover
 	 * the field, so don't get too fancy with the pre-processor!