SU can't connect. Why?

This one has to be simple, but I can't find a reason. I open a Konsole, run emacs, everything is fine. I enter su and the password, try to run emacs and get the following: Xlib: connection to ":0.0" refused by server Xlib: No protocol specified When I tried to run the mozilla 1.3.1 installer as su, I get the same error. It looks to this relative newbie that there is something configured that su doesn't have, but what's the missing piece?