Brown trout collected from fertile and infertile streams in Pennsylvania were compared to determine differences in fecundity and in the ago at first sexual maturity. Trout from infertile streams were older at first sexual maturity and produced fewer eggs because of smaller average size than trout from fertile streams. Fish of the same size produced a smaller total weight of eggs in infertile waters than in fertile waters. Sexual maturation depended on both size and age. It was concluded that in waters of low basic productivity few eggs are produced per adult trout in the stock and that this depressed rate of reproduction is important in keeping brown trout populations in equilibrium with the low basic productivity of the environment.