Laboratory experiments with thermal plumes in fluid with temperature-dependent viscosity suggest that wave-like instabilities can form in the horizontally flowing, disk-shaped head of the plume. The waves propagate radially outward from the axis of the plume and appear to be most intense in a finite band near the perimeter of the plume head. A simple theoretical model shows that interfacial waves in a highly viscous fluid may occur if the plume-head is comprised of temperature-dependent-viscosity fluid that cools as it flows between two boundaries. The model suggests that the waves arise as an oscillatory instability and that wave formation is most robust in the colder regions of the plume-head, as indicated by the experiments. The theory also predicts that the instability will only occur above some critical plume-head flow velocity, and that mantle plume conditions are generally supercritical. ¿ American Geophysical Union 1992