All male mules and most female mules are infertile

A mule is the offspring of a male donkey and a female horse.^[1] Historically, and today in technical jargon, "mule" refers to the infertile offspring of any two animals of different species. Horses and donkeys are different species, with different numbers of chromosomes. Of the two F1 hybrids between these two species, a mule is easier to obtain than a hinny (the offspring of a male horse and a female donkey). All male mules and most female mules are infertile.

http://en.wikipedia.org/wiki/Mule