Molluscs dated from 13 000 radiocarbon years B.P. to the present day were sampled at sites throughout southwestern Alberta. The ratio of southern forms (those presently occurring only south of 55°N) to northward-ranging species (those also occurring north of 55°N) in the major chronological sediment zones reflected the main climatic trends of the period. A study of present-day ecological preferences of the species allowed speculation on successional status of past terrestrial vegetation, agreeing satisfactorily with a previously published pollen diagram from central Alberta. Three major climatic periods could be identified: (1) a warming trend with precipitation higher than that of today from the beginning of the Laurentide ice retreat until about 9000 radiocarbon years B.P., (2) still warmer but drier conditions 9000 to 7000 radiocarbon years B.P., and (3) gradual cooling and drying until present-day conditions.