cannot su

First, I'd like to say my gentoo system has been progressing nicely.  I
only have a few small problems left.  The one of most concern is that I
cannot su to the root user.  When I am logged in in a terminal, I cannot
attain root priveledges with su, it just says 

su: Permission denied
Sorry

I know that the password is correct, because I log directly into a
terminal as root.  I just can't do it via su.

--Matt