Parasitic castration may result from manipulation of host energy allocation away from reproduction, which should result in castration of lightly infected hosts as well as heavily infected ones. Castration also may result from nutrient theft alone, which incidentally influences host energy allocation to reproduction and should cause reproduction to end in heavily infected hosts. Although the pseudophyllidean cestode Schistocephalus pungitii is a castrator of ninespine stickleback fish (Pungitius pungitius), the cause and significance of castration remain unknown. We used predictions about reproductive capacity in lightly and heavily infected hosts and host size (age) at which castration occurs to address these questions. In Airolo Lake, Alaska, we found that inhibition of reproduction in ninespine stickleback occurs in small fish before the size (age) of sexual maturity and is not instantaneous. Infected fish do not reach the same size as uninfected fish, possibly due to growth inhibition or selective predation. Castration in infected adult-size female ninespine stickleback appears to be the result of adaptive manipulation of host energy allocation by the parasite, but it could result from an energy budget readily influenced by parasitism.