The role of asymmetry of zonally and temporally invariant, equatorial, zonal flow on the structure and dynamics of tropical instability is investigated in a continuously stratified β-plane model. While more strongly developed flow in the northern hemisphere leads to a northern hemisphere enhancement of the instability, consistent with observational data, asymmetry is not found to dramatically change the internal dynamics of tropical instability. We find that both sides of the northern branch of the South Equatorial Current are potentially unstable, with the relative level of meridional shear on the two sides of the flow playing a role in determining the location of the dominant source region. Consideration of the structure of the energy conversions, however, demonstrates the strong dynamical importance of the surface of zero meridional, isopycnal, potential vorticity gradient in setting the phase speed and controlling the structure of the instability. This control can be clearly understood through the dynamical framework provided by the wave geometry construct. ¿ 1998 American Geophysical Union