The effect of thermal anisotropies on the Drummond-Rosenbiuth two-stream ion cyclotron instability is studied. The properties of the instability are found to be dependent on the ratio of parallel electron temperature to perpendicular ion temperature, T∥e /T⊥ ι which can alternatively be viewed as the product ofT∥e /T⊥ ι and the ion anisotropyT∥e /T⊥ ιThe critical drift velocity decreases and the growth rate increases with T∥e /T⊥ ι. However, the dependence on T∥e /T⊥ ι becomes less sensitive as this temperature ratio increases, and, in the limit of T∥e /T⊥ ι>1, the instability becomes independent of the perpendicular temperatures.