Scott Cate just posted an interesting usage to get Fiddler to work on localhost here Working with Fiddler on LocalHost. The short of it is to use http://127.0.0.1 but add an extra period to the end of it as in http://127.0.0.1. and if working on Cassini add the port number (with the colon) after that last period. I recall having this problem in the past and now I have a solution for it. Thanks Scott!