I have 3 cats and they occasionally do this too.  Compared to other problems
in life, this is relatively minor.

--
Barb
Of course I don't look busy,
I did it right the first time.

Try adding a 2nd litter box.  One of my cats is funny...he likes to pee in
one and poop in the other.  Try a second box and see what happens.

S.

The suggestion for another litter box is a good idea.  Many cats "require"
more than one box.  Also, here's another possibility:  Are you sure that
this is not feces that clings to the fur and then drops out after your cat
exits the litter box?  This is also very common, especially with long-haired
cats.  Some people even trim that area to reduce the possibility.

MaryL

This is not an everyday problem.  I change the litter box very often.
I do have another cat which I have had for 6 years and she has never
done anything like this.  What to do?  HELP!!!!

I would mention it to your vet next time you go . He might be
sometimes get constipated or have a problem with his anal glands.
  Alison

Does Tux have long hair on his rear? (I know, that's a personal
question.) We have one medium-haired cat who does this once in awhile
just because it gets stuck on his rear.

If not, another litter box (or 2!) is a great idea.

Rhonda

He might need his behind clipped, if it's clinging to his hair
and dropping off outside the box.  

It you think the problem might be his butt overhanging the edge
of the box, you should see about getting a mortar-mixing tray for
a litter box.  They cost about $4 from Home Depot.  It solved my
problem of one cat pooping over the edge and the other managing to
pee over the corner or end or the litter box.

These are very sturdy plastic like a litter box, about 20" x 28"
and about 7" deep.  The sides slope outwards more than a litter box,
so the cats can back up to the ends or corners of the box and still
have their behinds contained in the box.