The instability of an arbitrarily shaped zonal jet on a midlatitude&bgr;-plane is considered within a two-layer quasi-geostrophic model withO(1)linear friction. Depending on the horizontal and vertical shear of the jet, it is susceptible to both barotropic and baroclinic instabilities. The linear stability boundaries are determined numerically for a parameter regime relevant to the Gulfstream. The weakly nonlinear (finite amplitude) evolution of the instabilities is shown to be governed by a Ginzburg-Landau equation and for arbitrary jet shapes the coefficients in this equation are computed numerically. The finite amplitude state is shown to become unstable to Benjamin-Feir sideband instabilities. The mixed baroclinic/barotropic character of the primary instability is crucial to this sideband instability which is shown to lead to complicated spatio-temporal behavior of the jet. ©1997 American Institute of Physics.