Shell morphology and sculpture, population size structure, sediment grain size preference, and the reproductive cycle, are described for the common estuarine mud flat gastropod, Amphibola crenata (Martyn 1784). Snails which occurred near to sewage outfalls in the Avon-Heathcote Estuary, are compared with those which occurred In an area with similar salinity and exposure conditions but isolated from possible effects of sewage. Sediment silt and organic carbon content were higher near to the outfalls. Shell morphology, described by shell dimensions, geometrical parameters of coiling, linear weight relationships and shell deposition patterns, showed no significant difference between the two areas. A predominantly juvenile population which occurred near to the sewage outfalls was the result of an annual cycle of settlement and sub-sequent population loss. A predominantly adult population occurred in the area unaffected by the outfalls. In laboratory experiments, animals from each area preferred sediment from their own particular area. Size frequency and sculpture distribution showed no regular zonation patterns for other areas of the estuary. Seasonal changes in the ovotestis were not significantly different between animals collected from the two areas. Tissue levels of heavy metals in A. crenata collected from near to the sewage outfalls were lower per unit tissue dry weight but higher in total metal per individual than in those from the other area. Tissue and sediment levels of metals were relatively low in both areas compared with levels reported in studies of polluted conditions. Isolated areas of elevated metal levels occurred in the Avon River. Exceptionally long survival times for A. crenata exposed to copper(ii) ion concentrations in the laboratory, are attributed to pulmonary respiration, the operculum, and high mucous production. Salinities below 8% seawater and increase in temperature affected the toxicity of copper to adults. Veliger larvae were sensitive to much lower concentrations of ionic copper(ii) (0.5ppm) than those which affected egg development (5ppm) or survival of adults (2.5ppm). The size distribution of A. crenata near to the sewage outfalls does not seem to be a result of the presence of toxic materials in the sewage.