BackButton when going from a fragment to load a new activity

Here is my situation: 1. I am in Activity A1. 2. Activity A1 loads fragment F1, which is a list fragment. 3. when I select one of the items in the list fragment F1, we load a new Activity A2. 4. When I hit the back button while in A2, the application exits.

I've tried multiple calls to the fragment transaction's .AddToBackStack().

I would think that going back from A2 would take me to A1. Unfortunately, the back button is exiting the application. Admittedly, the application has had several of us working on it. There may be several things happening that I am not sure about. Any suggestions on how to go from A2 back to A1/F1?

Wally