By using a five pass Fabry-Perot interferometer we have observed Brillouin scattering from longitudinal sound waves in films of amorphous silicon prepared by glow discharge decomposition of silane. Both the sound velocity and the scattering efficiency are found to depend on the hydrogen content of the film. For small hydrogen content the velocity is observed to be comparable with that of crystalline Si. The implications of our results for pure amorphous Si are different than for the results of Testardi and Hauser on sputtered samples. © 1978.