I usually use the gdm for X session manager. Under this circumstance, I wanted to test a xdm, a very traditional X session manager. Rather than replacing gdm with xdm, I tried to start a xdm within a gdm session. Here is how I did: In a gnome-terminal type as follows: # su # xdm -server ":1 nest local /usr/bin/Xnest :1 -geometry 800x600" Cheers!