Can't find library error

[luisgh@cogs.susx.ac.uk (Luis Francisco Gonzalez)]

Hi,
I am expiriencing a problem that I don't know how to solve.

masca[~]$ xtet42 &
[1] 769
masca[~]$ xtet42: can't find library 'libX11.so.6'

The problem is that the library is there:
masca[~]$ ls -l /usr/X11R6/lib/libX11*  
lrwxrwxrwx   1 root     root           13 May 10 15:43 /usr/X11R6/lib/libX11.so -> libX11.so.6.0
lrwxrwxrwx   1 root     root           13 May  9 16:29 /usr/X11R6/lib/libX11.so.6 -> libX11.so.6.0
-rwxr-xr-x   1 root     root       591290 May  7 16:03 /usr/X11R6/lib/libX11.so.6.0

masca[~]$ dpkg -S libX11
xdevel: /usr/X11R6/lib/libX11.so
xlib: /usr/X11R6/lib/libX11.so.6.0

masca[~]$ dpkg -s xlib  
Package: xlib
Status: install ok installed
Priority: standard
Section: x11
Maintainer: Stephen Early <sde1000@debian.org>
Source: XFree86
Version: 3.1.2-7
Replaces: elf-x11r6lib
Provides: xR6shlib, xlibraries, elf-x11r6lib
Depends: libc5, ldso (>=1.7.14-1)
Conflicts: elf-x11r6lib
Description: XFree86 3.1.2 shared libraries

I am missing something or should this work?

TIA,
Luis.