Anecdotal reports of fertility in female mules (jack donkey × mare) and hinnies (stallion × jenny donkey) have appeared in the literature over the years, but scientists have generally regarded them with scepticism. The fact that some of these hybrids can come into estrous and ovulate makes fertility conceivable, given that opportunity for mating arises. In China, where mules are bred extensively for work on the farms, a fertile female mule and a fertile female hinny have now been verified by chromosomal investigation. Each had mated with a donkey and produced a filly foal. The foals show unique hybrid karyotypes different from the mule’s or hinny’s and different from each other’s. The studies make it clear that mule and hinny fertility, at least for the female hybrid, is a real possibility.