Abstract It has been suggested that hermaphroditism may evolve when the resources that females can profitably allocate to ova is limited by factors such as lack of brooding space. Spare resources could then be allocated to produce male gametes in a hermaphrodite. A model is developed to investigate the conditions under which this will occur. Hermaphrodites will displace males (and females) if the hermaphrodites produce at least half as many male gametes as a male. If the hermaphrodite produces less than half the number of gametes produced by a male then a stable equilibrium arises where males and hermaphrodites coexist. In this situation the frequency of males is determined by the ratio of the numbers of male gametes produced by hermaphrodites to the numbers produced by males.