I have about ten or twelve rosy minnows that have been in my pond for about a month, and the same number of fathead minnows in another smaller pond. I was hoping they would come into breeding condition but when I fished them out and looked at them, they appeared to all be female (no males i.e. no tubercules, dark coloration, all the fish had ovipositors)
Is it possible that I got two dozen female minnows? Or are they too immature to sex yet? I bought them from Petsmart hoping to get some breeding stock, but maybe they only ship females?

It's highly unlikely you have all of one sex although possible. They may simply not be mature enough. You haven't had them that long. Males aren't always obvious if they're not "in the mood." Even a male in his prime will "bleach out" when picked up for inspection, and his black coloration may be faded from the stress. Males have what looks like a smaller ovipositor but it's just their opening (vent) but it does stick out a little. My newest batch of male rosy reds, although mature last year when I put them in my pond (at least one batch of fry), did not set up semi-permanent nests that they defended until this year. Now, I have babies galore. If your minnows are healthy, you will have babies but maybe not until next year.

Thanks for your quick reply, it took care of the problem. It's too bad I have to wait another year (patience is not my best known attribute) but I'll let ya know if and when the rosies spawn. Thank you again for your help!

Good news! Today I spotted a large rosy minnow squirming with another smaller one which I immediately recognized as spawning. Taking a seat, I watched for a half an hour as they spawned against a plastic flower bowl. I lifted up the pot once to see the jelly-like eggs underneath.
I know that once the fry hatch, it'll be lunchtime for all the residents of the pond... should I try artificially rearing the fry? (I have experience with this)
And if I remove the flower bowl from the pond, how long should i keep the male with the eggs?

Thanks for your help, I'm very excited!

Some fry will survive depending on the size of your pond, how many places they have to hide, the food supply, etc. I have a bumper crop of rosy red babies this year even with all the predators around. My site at http://www.fishpondinfo.com/rosy2.htm has a section on raising the eggs and fry. If you want a larger amount to survive, remove the pot right after the eggs are laid into another pond or aquarium. While the male does eat off fungused and infertile eggs and fans the eggs, females will come in and eat some eggs. If you want to keep them with him and get them right before they hatch, you can but it's hard to time it just right. Once they hatch, they're fair game as far as being eaten by the parents and other animals plus they're so small it makes them hard to catch. If you take his pot away, be sure to give him another right then.