A numerical model of a baroclinically unstable jet in a zonally periodic channel is used to analyze mesoscale eddy fluxes and their relationship with the gradients of the mean flow. The quasi-geostrophic approximation proves the best way to calculate potential vorticity fluxes in the primitive equation model. Away from the surface layers, eddy fluxes of potential density are consistent with advection by eddy-induced velocities v* and w* as suggested by Gent et al. (1995). Eddies mix potential vorticity along isopycnals, so that v* is related to the gradients of potential vorticity rather than potential density as implicitly assumed by Gent et al. The mixing coefficient for potential vorticity, associated with the advective component of the eddy fluxes, is found to be similar to the mixing coefficient of tracer anomalies on isopycnals. Both show a maximum at mid-depth below the jet core. The present calculations support the analysis of Treguier, Held and Larichev (1997) and encourage further attempts to derive a parameterization based on true mixing of potential vorticity.