diff --git a/Doc/lib/libftplib.tex b/Doc/lib/libftplib.tex
index dfbaa2be071..a89c6a9ecb8 100644
--- a/Doc/lib/libftplib.tex
+++ b/Doc/lib/libftplib.tex
@@ -125,14 +125,15 @@ Return nothing if a response code in the range 200--299 is received.
 Raise an exception otherwise.
 \end{funcdesc}
 
-\begin{funcdesc}{retrbinary}{command\, callback\, maxblocksize}
+\begin{funcdesc}{retrbinary}{command\, callback\optional{\, maxblocksize}}
 Retrieve a file in binary transfer mode.  \var{command} should be an
 appropriate \samp{RETR} command, i.e.\ \code{"RETR \var{filename}"}.
 The \var{callback} function is called for each block of data received,
 with a single string argument giving the data block.
-The \var{maxblocksize} argument specifies the maximum block size
-(which may not be the actual size of the data blocks passed to
-\var{callback}).
+The optional \var{maxblocksize} argument specifies the maximum chunk size to
+read on the low-level socket object created to do the actual transfer
+(which will also be the largest size of the data blocks passed to
+\var{callback}).  A reasonable default is chosen.
 \end{funcdesc}
 
 \begin{funcdesc}{retrlines}{command\optional{\, callback}}
diff --git a/Doc/libftplib.tex b/Doc/libftplib.tex
index dfbaa2be071..a89c6a9ecb8 100644
--- a/Doc/libftplib.tex
+++ b/Doc/libftplib.tex
@@ -125,14 +125,15 @@ Return nothing if a response code in the range 200--299 is received.
 Raise an exception otherwise.
 \end{funcdesc}
 
-\begin{funcdesc}{retrbinary}{command\, callback\, maxblocksize}
+\begin{funcdesc}{retrbinary}{command\, callback\optional{\, maxblocksize}}
 Retrieve a file in binary transfer mode.  \var{command} should be an
 appropriate \samp{RETR} command, i.e.\ \code{"RETR \var{filename}"}.
 The \var{callback} function is called for each block of data received,
 with a single string argument giving the data block.
-The \var{maxblocksize} argument specifies the maximum block size
-(which may not be the actual size of the data blocks passed to
-\var{callback}).
+The optional \var{maxblocksize} argument specifies the maximum chunk size to
+read on the low-level socket object created to do the actual transfer
+(which will also be the largest size of the data blocks passed to
+\var{callback}).  A reasonable default is chosen.
 \end{funcdesc}
 
 \begin{funcdesc}{retrlines}{command\optional{\, callback}}
diff --git a/Lib/ftplib.py b/Lib/ftplib.py
index ec7d4791ddb..024a0faea4b 100644
--- a/Lib/ftplib.py
+++ b/Lib/ftplib.py
@@ -317,7 +317,7 @@ class FTP:
 			raise error_reply, resp
 		return resp
 
-	def retrbinary(self, cmd, callback, blocksize):
+	def retrbinary(self, cmd, callback, blocksize=8192):
 		'''Retrieve data in binary mode.
 		The argument is a RETR command.
 		The callback function is called for each block.