The age at which the male elk becomes sexually mature has not been known. Reports in the literature are based upon instances of observed copulations rather than upon testicular anatomy. In these observations of breeding activity there is considerable disagreement as the following statements indicate. West (1941) says "bulls usually do not become of breeding age until they are 3' years of age." However in the same paper he reports that in 1935 yearling bulls were observed breeding in the Gallatin National Forest. Green (1933) states that "2 or 3-year olds, although desirous and capable of performing the sex act, stand little chance in open competition with their elders." Murie (1951) records an observation of a yearling male breeding but concludes that, "On the whole although spike bulls occasionally may be found somewhat active sexually they are not so normally." The present study is concerned with the age at which spermatogenesis occurs in the elk rather than the age at which breeding begins. It is of interest to know the age at sexual maturity so that the significance of observed reports of copulation may be better understood.