Selenium was determined in duplicate portions from three bottles of six U. S.G.S. standard rocks by a spectrofluorimetric procedure. The following averages, as ppm Se, were obtained: PCC- 1, 0 031; GSP-1, 0.988; BCR-1, 0.12; SCo-1, 0.91; MAG-1, 1.3; and SGR-1, 3.7. One-way analysis of variance of the several sets of data showed no significant differences in the selenium content among bottles of any specific rock; these samples may be accepted as homogeneous for their selenium contents by this analytical method.