Within the Stratosphere Troposphere Experiment by Aircraft Measurements (STREAM) project, airborne measurements of various trace species such as CO, CO₂, N₂O, and O₃ have been performed in several years and seasons. Here we focus on a summer campaign conducted from Timmins (Canada, 47°N, 80°W) in July 1998 and winter measurements carried out in March 1997 from Kiruna (Sweden, 68°N, 20°E). From correlations between CO and O₃ we identify a mixing layer in the lowermost stratosphere of which the vertical extent shows a significant seasonal variability. Troposphere to stratosphere exchange appears to be more vigorous in summer, although CO mixing ratios of the order of 15 ppbv indicate the presence of aged stratospheric air. While a significant tropospheric influence was observed up to potential temperatures of Θ = 330 K in March, the mixing layer in July increased to Θ = 360 K. In addition, its elevation above the tropopause also showed an increase. The CO-O₃ correlation indicates that the higher elevation in July is most likely caused by a stronger contribution of subtropical tropospheric air, which was mixed into the lowermost stratosphere at the subtropical jet.