There are several things that can cause green stool:
-iron pills
-vitamins with chlorophyll
-too much bile passing thorugh your digestive system
-foods passing too quickly
-food coloring
-eating lots of green, leafy vegetables
All these are harmless. If none of these applies to you, you may want to see your doctor.

I had green stools once when I was having issues with my intestine. MY GOSH it was like bright green!! NOt a nice thing to see.

My dr, treated me for my intestine issues, and the green stools went away. He didnt seem to worried about the green though, I was more worried about it then he was. He just oh ya, wow it really is green ...um ya lol

But for sure I would just talk to your Dr about it, but I think deteragram is right that it most likly is harmless so dont stress over it, just ask the dr.