The generation of secondary vorticity in maintained and temporally-decaying wave motions is investigated for internal waves in fluid of great depth. In the case of two semi-infinite, homogeneous fluids of different density, the interfacial boundary layers generate a second-order, mean vorticity which diffuses inwards into the interior of both fluids, and the net vorticity produced is zero. For a continuously-stratified fluid, the free surface layer plays an indirect role and secondary vorticity, initially generated only within stratified regions by the action of a Reynolds stress, diffuses, in general, over the whole fluid, and no steady-state vorticity field is established. In finite depths, a steady state ultimately exists for maintained waves, and the mass transport velocity field is investigated.
