It was a caterpillar. It had its snack and left.

If left on the vine, that tomato would have ripened okay. Then, to eat it, you just trim out the hole.

For an organic product, you can use BT (Bacillus thuringiensis kurstaki). It kills caterpillars (though they may munch for a day or so before dying) and won't bother anything else. BT is available in various garden products. [You don't want any BT product that isn't BT kurstaki. The other strains of BT kill other things but don't affect caterpillars.]

(And BT kurstaki kills caterpillars but won't kill caterpillar-like creatures like the pine sawfly larvae currently infesting my dwarf mugo pines; I go on patrol daily. Isopropyl alcohol is marvelous stuff.)

I agree with the above and most likely suspect IMO would be a tomato fruit worm.

But I will add that others have posted similar pics lately and some folks have suggested bird damage. I have never personally had a problem with birds pecking my fruit but it does make sense to me that a bird could do that type of damage.

Dave

thanks, everyone, i'm ordering greenlight BT but can't tell if it is BT kurtaski

http://www.amazon.com/gp/product/B000UGPSMY/ref=oh_details_o01_s00_i00

Why not just go to the the maker's website (Greenlight) and look it up. It should tell you there since they have to post the MSDA info.

Dave

thanks, it was such small print, but yes, green light BT is kurstaki

Yes, indeed. The clue is that it says "worm killer"

Beyond that, if the hole was due to birds, it would be angular, not round. (Gotta pay attention to the critters' eating patterns,)

came back to tomatoes again yesterday afternoon and caught it snacking on another! a green worm?

I believe that's a tomato fruitworm (same as corn earworm). The coloration varies, and the older ones have stripes. Here's a photo of a bunch of them with different coloration:
http://entoplp.okstate.edu/ddd/insects/cornearworm.htm

They mostly make deeper holes, frequently very messy.