A palynological study supported by radiocarbon dates of late-glacial sediments at St. Hilaire, Quebec, indicates that the southern part of the St. Lawrence Lowland was deglaciated prior to 12 500 years B.P. The late-glacial episode comprises several climatic fluctuations: a probable early cool interval (northern boreal) more than 12 500 years B.P.; a relatively colder interval (tundra) about 12 500 years B.P., followed by another cool interval from about 12 000 years B.P. to about 10 000 years ago. Another relatively cold episode may have occurred about 11 000 years ago. The new studies extend the previously available palynological record in the St. Lawrence Lowland back in time by about 2000 years and include the Champlain Sea episode.