The objectives of this study were to determine whether there are seasonal changes in spermatogenesis for pre-pubertal and post-pubertal grizzly bears (Ursus arctos horribilis), and investigate the seasonal association between testis mass and serum testosterone (T) concentrations for post-pubertal grizzly bears from the continental US from May through October. Testes from 25 grizzly bears were collected from bears killed by federal and state wildlife personnel in Montana and Wyoming from 1978 to 1995. Fifty blood samples were obtained from wild, post-pubertal (≥5.5 years) male grizzly bears from May through October in Montana and Wyoming from 1993 through 1995. In pre-pubertal bears, the seminiferous tubules were small and surrounded by abundant interstitial tissue in May. Tubules were enlarged and closely packed July through September. Tubules began to degenerate in November. Although spermatogonia and spermatocytes were present from May through September, spermatids never occurred within seminiferous tubules. The epididymal tubules in pre-pubertal bears were well organized from May through September, although they never contained spermatozoa. In post-pubertal bears, spermatogenesis changed seasonally: the entire spermatogenic population from spermatogonia through spermatids was present May through August, and spermatogonia and spermatocytes were present in October and November. The seminiferous epithelium began to deteriorate in July. The epididymal tubules contained spermatozoa May through August only. Both testis mass and T concentrations peaked in June. Mean T concentrations during May and June were greater (P = 0.02) than those during July through October. These results suggest that in grizzly bears in the continental US, seasonal changes in spermatogenesis are accompanied by changes in testis mass and T concentrations and both are associated with photoperiod.