The changes in the real and imaginary parts of the index of refraction of germanium have been measured at 6943 A as a function of incident power up to 5 MW/cm(2). The results indicate that for nanosecond pulses the primary cause of the change is heating with temperature changes of up to 350 degrees C. In addition, a diffraction grating formed on the surface by interference between the intense beam and a probing beam produces an additional effective change in reflectivity as well as a spatially separated diffracted beam. It is shown that, for a grating spacing of 20 mum, transverse diffusion effects cause no degradation in the effectiveness of the grating.