Experiments have been carried out to determine whether cross–fertilization, multiple insemination and sperm sharing occur in Bulinus cernicus. Snails collected from Canal La Paix, Mauritius, were found to be polymorphic for glucose phosphate isomerase, and from this stock three snail colonies homozygous for GPI 2/2, GPI4/4 and GPI6/6 were selected. Thus, three markers were available to monitor cross–fertilization. Isolated snails produced egg–masses by self–fertilization 40 days post–hatching. Egg–masses produced by self–fertilizing individuals tended to be smaller than those from cross–fertilizing snails, 367 eggs per egg–mass as opposed to 5–34. The majority of snails (70%) switched rapidly to cross–fertilization after pairing. Multiple insemination by two partners was common, heterozygous progeny were produced in a random manner and did not reflect the sequence of mating. No evidence of sperm sharing was found, snails were found to be capable of passing on their own sperm while still producing eggs fertilized by sperm received from an earlier mating. Differences in mating behaviour were noted between snails representing the homozygous GPI colonies, with snails from one colony adopting the female role more readily than those from the other two, which predominantly mated as males.