Laboratory experiments have been conducted to examine the effects of rotation on the structure and stability of hydrothermal plumes. It is shown that the plume forces a baroclinic vortex, consisting of an anticyclonic eddy at the neutral buoyancy level and cyclonic circulation in the ambient fluid around the rising plume. These features agree qualitatively with Speer's (1989) model. However, the experiments show that this state is unstable, leading to the breakup of the plume and the unsteady generation of baroclinic vortices (hetons) that propagate away from the source. Two nearby plumes interact repulsively for separations >NZs/f, and initially coalesce for smaller separations. Here N is the buoyancy frequency, f is the Coriolis frequency, and Zs is the height of the spreading level. It is suggested that oceanic hydrothermal plumes may shed isolated baroclinic vortices, which are capable of retaining anomalous properties (temperature, salinity, 3He, etc.) over large distances. ¿ American Geophysical Union 1991