Local mate competition theory has emphasized the importance of sperm competition in determining individual patterns of sex allocation in outcrossing hermaphrodites. In this article I develop a more general model of local mate competition that incorporates fertilization efficiency. I use this model to predict interspecific differences in sex allocation. These predictions are tested by using dry weights of male and female gonadal tissue from six species of hermaphroditic sea basses. In all four paired comparisons, when two of the three factors presumed to determine sex allocation patterns are relatively constant, the shift in sex allocation between species is correctly predicted. Neither levels of sperm competition nor the expected effects of spawning behavior on fertilization efficiency can account for all of the differences in sex allocation, although sperm competition does correctly predict the most differences in sex allocation. Overall, the interspecific differences are predicted by either the level of...