An electromagnetic wave propagating obliquely to a counterstreaming plasma is shown to be unstable for all directions of the stream. The stream direction normal to the wave vector is a singular case for which the growth rate increases from zero as the wave number is increased and asymptotically approaches a maximum value for very large wave numbers. For all other directions unstable waves exist for wave numbers below a critical value and the growth rate increases from zero as the wave number is increased, reaches a maximum value, and then decreases to zero at the critical wave number. The maximum growth rate is found to be the same for all stream directions other than very close to the normal to the wave vector. The dependence of the growth rate on the stream direction with respect to the wave vector and the validity of the quasistatic approximation in predicting the nature of this instability are also examined.