The instability of an electromagnetic wave propagating obliquely to a counterstreaming plasma is investigated for the case in which the magnetostatic field is normal to the plane containing the wave vector and the stream direction. The stream direction normal to the wave vector is a singular case for which, if the magnetostatic field is less than a critical value, the instability occurs for wave numbers exceeding a minimum value, the maximum growth rate being attained for very large wave numbers. An increase in the magnetostatic field increases the minimum wave number and reduces the growth rate. For all other stream directions unstable waves exist between two limiting wave numbers for all values of the magnetostatic field. The maximum growth rate is found to be practically the same for all stream directions other than normal to the wave vector. An increase in the magnetostatic field reduces the range of wave numbers for which there is instability, causes the region of instability to shift to shorter wavelengths, and decreases the maximum growth rate. 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.