Three types of the cryptorchid condition are described, nullitesticular, unitesticular and the usual form with two testicles where one or both are retained in the coelom. The nullitesticular condition was observed in only one animal. The unitesticular condition was verified by autopsy in 26 of 122 cryptorchids studied. Twenty-one of the unitesticular boars had two normally descended testicles at birth but one had disappeared from the scrotum after 42 days of age. The remainder (five) were unitesticular at birth.Selection studies were conducted with two breeds to provide genetic information on the cryptorchid condition. In the Lacombe breed, five sires cryptorchid at birth produced 33 litters with an average incidence of 10.9% cryptorchids among their male progeny, whereas 21 cryptorchid sires which were normal at birth produced 129 litters with 3.8% cryptorchid males. In the Yorkshire breed, four boars cryptorchid at birth produced 25 litters with an average incidence of 31.4% cryptorchid male progeny. The practical significance of these findings is discussed.