Measurements of ozone number mixing ratio made in the mean troposphere, both within and above the boundary layer, at 60 ¿N latitude over Canada in the summer of 1977, range from 200 ppb. Isentropoic analyses of the dynamic tropopause, on the basis of a potential vorticity threshold, show that the large-scale anomalies are related to traverses of a multiply folded tropopause; therefore, the large values of ozone are of stratospheric origin. Values <30 ppb are shown to be made directly in the boundary layer or in moist air which can be traced back to a boundary layer. The distinctions between the conventional and dynamic tropopause are discussed, and the use of potential vorticity as a stratospheric tracer is defended.