Wayne,

Jenluckenbach is right, the rule of thumb is 1 box for every cat (although they will ALL use each others). The best thing to do is confine the little one in a room with her food and water and the cat box. That's what my vet told me when my oldest cat began pooping behind my chair in the living room. It worked!

It could also be the cat litter. Cats don't like change.

How old is kitty??

Good luck.