ABSTRACTA study of long-term temperature data (1881–1970) from 13 stations in Canada, east of the Rockies, shows that the mean length of the annual frost-free season decreases zonally from east to west. Meridional distinction among the stations though present is muted. Three regions are identified—a prairie region, a southern Ontario-South Atlantic axis and a Quebec-Maritime region. The spatial correlation in the length of the season is strong to moderate within regions, but declines rapidly between regions. A strong internal consistency in the length of the season at each location is observed. However, most stations show a marked change in the nature of the season after 1940 which resulted in a significant shift in the probable dates of the first and the last frost of the season.