We show that the equation of quasigeostrophic (QG) potential vorticity conservation in geophysical fluid dynamics follows from Hamilton’s principle for stationary variations of an action for geodesic motion in thef-plane case or its prolongation in the&bgr;-plane case. This implies a new momentum equation and an associated Kelvin circulation theorem for QG motion. We treat the barotropic and two-layer baroclinic cases, as well as the continuously stratified case. ©1998 American Institute of Physics.