diff --git a/Doc/lib/libstrings.tex b/Doc/lib/libstrings.tex
index e9b0bcb..7294002 100644
--- a/Doc/lib/libstrings.tex
+++ b/Doc/lib/libstrings.tex
@@ -24,4 +24,7 @@
 \item[StringIO]
 --- Read and write strings as if they were files.
 
+\item[cStringIO]
+--- Faster version of \module{StringIO}, but not subclassable.
+
 \end{description}
diff --git a/Doc/libstrings.tex b/Doc/libstrings.tex
index e9b0bcb..7294002 100644
--- a/Doc/libstrings.tex
+++ b/Doc/libstrings.tex
@@ -24,4 +24,7 @@
 \item[StringIO]
 --- Read and write strings as if they were files.
 
+\item[cStringIO]
+--- Faster version of \module{StringIO}, but not subclassable.
+
 \end{description}