Estimates of the net vertical flux of ozone in the lower stratosphere were made on the basis of the net vertical flux of potential vorticity in the lower stratosphere in two general circulation model runs. Potential vorticity fluxes, calculated from the January and the July simulations, were combined with observed ozone distributions to estimate average net vertical fluxes of ozone of 0.49 and 0.25¿1011 molecule cm-2 s-1 for the northern and the southern hemispheres, respectively. This suggests a hemispheric asymmetry in the vertical transports, although it is insufficient to account for all the asymmetry in the tropospheric ozone distribution. Combined with surface deposition fluxes for ozone of 0.82 and 0.48¿1011 molecule cm-2 s-1 for the northern and the sourthern hemispheres, respectively, these net vertical fluxes of ozone in the lower stratosphere support the hypothesis of a hemispherically asymmetric photochemical source of ozone in the troposphere.