A study of epiphytic gastropods associated with two submerged macrophytes (Myriophyllum spicatum and Vallisneria americana) was conducted at a soft and at a hard water site in the St . Lawrence River during two non-consecutive years in order to compare effects of macrophyte species and biomass, site and year in gastropod community structure . There was no effect of macrophyte species on total gastropod abundance nor on gastropod diversity, and few gastropod species showed a marked preference for either macrophyte species . Inter-site and inter-annual variations in diversity, total gastropod abundance and gastropod community structure were greater than variations among macrophytes . However, analysis of variations of epiphytic gastropod communities in one of the two sites shows that abiotic factors are important in explaining epiphytic gastropod distribution . Our results and results of other studies on gastropod population dynamics in the St . Lawrence River indicate that intra-and interspecific competition between gastropods is important on macrophytes and that they must be carefully considered in order to understand epiphytic community structure and population dynamics .