The acorn barnacle, Semibalanus balanoides is a sessile, simultaneous hermaphrodite with a large and extensible penis, making it an excellent model system for the study of reproduction in hermaphrodites. Several different factors affecting the likelihood of reproductive success were measured: neighbor distance, body-size, and penis length. We used regression methods to determine the extent to which these factors influenced the reproductive success and found that neighbor distance and body size were the most important factors in determining the probability of fertilization. Penis length was also an important component of mating success and was closely linked to body size. This data can be used to help infer the viability of expanding or contracting populations of acorn barnacles like S. balanoides.