Mean seasonal species composition of living molluscan communities was compared with the composition of current dead assemblages in the sediments of three sites located in the Delta Marsh of southern Lake Manitoba. Dead shells were more numerous in vegetated than in bare areas, resulting primarily from the affinities of living molluscs for vegetated areas. Redistribution patterns of empty shells were not significantly different for vegetated and bare areas, as judged from distributions of passively transported land shells in the sediments. Significant differences were observed at all sites between species frequencies in living and corresponding dead assemblages averaged for the season. Proportions of living to dead individuals per unit bottom area indicated higher attrition rates with increasing energy conditions as well as with increasing shell size. Differential attrition may result in overrepresentation of small species in fossil assemblages.