Experiments were carried out to determine the effects of population density on the natality of Musculium securis (Prime). Laboratory studies showed that: (1) total recruitment increased with increasing densities of parental stock, to an asymptotic value, (2) that recruitment was greater than the reproductive carrying capacity, and (3) that the total litter size per average reproductive adult decrease with increasing densities of parental stock, to an asymptotic value. It is suggested that surplus recruitment may be an important mechanism in population control. Field studies supported these findings and showed that interspecific competition between Musculium securis and Musculium transversum resulted in lower natalities for both species. Also, competition among individuals of the predominant species is primarily intraspecific and the presence of the subdominant species has little effect on their reproductive capacities. Temporary coexistence is possible for the 2 species but Musculium transversum has a higher natality and probably would eventually exclude Musculium securis when they were complete competitors.