Semibalanus balanoides is a simultaneously hermaphroditic acorn barnacle that mates with its neighbors using a long, agile penis. Sex allocation theory (Charnov 1980, 1982) predicts that simultaneous hermaphrodites maximize total reproductive success by increasing allocation to the male role towards an asymptote at 50% as competition (between functional males) for eggs increases. Acorn barnacles present an ideal system testing this theory, because they are sessile and therefore limited to mates within reach of their penises. Offspring are easily collected, as they are brooded for several weeks after mating. I created experimental groups of barnacles with small or large numbers of potential mates. I allowed these to grow and develop egg-masses and testes specific to their mating group size. Before the brief period of mating activity (in the fall), I reciprocally transplanted individual barnacles from large groups to small groups (and vice versa). At the same time, I retrieved un-mated barnacles from similarly sized groups to measure allocation to male and female function. After the mating activity had ceased, I collected all of the experimental mating groups and measured total reproductive output for the focal individuals. I calculated fitness gained as a female by counting the larvae in each focal individuals brood. I calculated fitness gained as a male by genotyping several highly variable microsatellite loci for a subset of each neighbors brooded larvae and comparing those to potential parents. This allows a comparison of total reproductive fitness of experimentally transplanted barnacles (mating in groups for which they have not strategically allocated energy to the sex roles) with un-transplanted control barnacles. This experiment provides a critical test of sex allocation theory and demonstrates the dynamics of mate competition for sessile, copulating simultaneous hermaphrodites.