AbstractThere are relatively few gorilla births in North America each year, and the birthrate shows considerable fluctuation. A questionnaire returned by 48 of 52 North American zoos and biomedical institutions known to keep gorillas revealed that only 21% of the adult males sired offspring in 1978 and 1979. The viability and genetic diversity of the captive North American population is therefore threatened. Many males are organically sterile, and there is an inferential suggestion of sterility among females. Environmental and behavioral factors are also operative in reproductive failure. Raising and maintaining gorillas in pairs decreases the probability that they will copulate.