In some populations, female threespine sticklebacks (Gasterosteus aculeatusL.) frequently eat conspecific eggs, even when other food is abundant. We tested the hypothesis that egg-eaters have a fitness advantage relative to non-egg eaters by manipulating diet. Our results indicate a clear reproductive advantage for the cannibals. Females that were fed conspecific eggs and a mixture of other foods produced 1.8 times as many eggs over the breeding season as those fed a non-egg diet. Furthermore the cannibals were in better condition at the end of the breeding season than the non-cannibals. We suggest that egg-eating females gain access to nutrients that are lacking or rare in other foods items and/or that eggs are more easily digested than other food.