The clam shrimp Eulimnadia texana has a rare mating system known as androdioecy, in which males and hermaphrodites co-occur, but there are no pure females. In this species, reproduction takes place by outcrossing between males and hermaphrodites, or by selfing within a hermaphrodite. This system provides a unique opportunity to examine the adaptive significance of outcrossing and selfing in a single animal species. This study was designed to test an assumption and estimate a parameter of a model developed by Otto et al. (1993), which predicts the conditions for stability of the mixed mating system in E. texana. Specifically, we tested the assumption that the rate at which hermaphrodites encounter males is proportional to male frequency in a population, and estimated the proportion of encounters resulting in outcrossing (which is assumed to be constant). We found that encounter rates per hermaphrodite are proportional to male frequency, but that this relationship did not validate the low encounter rates assumed by the Otto et al. (1993) model. In addition, several social factors (male frequency, density of individuals, and interactions between age, density, and male frequency) affect the proportion of sexual encounters that result in outcrossing. The Otto et al. (1993) model, therefore, needs to be refined to take these results into account.