ANSWER:

It looks like you are hard-coding your shell in the .screenrc to bash, is that your logon shell as well? Try commenting out that line and trying.

Failing that, compare the value of your $PATH from outside of tmux/screen, from inside screen, and from inside of tmux? If nothing jumps out from there, compare the output of ‘env’ from all three setups.

by Mike Parson from http://unix.stackexchange.com/questions/37169