Crude extracts prepared from four Micrococcus varians strains, 11 M. luteus strains and four laboratory isolates subsequently classified with M. luteus were assayed for catalase activity following electrophoresis on polyacrylamide gels. The enzyme patterns produced from the M. varians strains exhibited three catalase isoenzymes which were distinguished into two types of patterns depending upon the location of the major band. The extracts from all the M. luteus strains produced the same pattern, composed of two catalase isoenzymes of similar electrophoretic mobility. For both species the isoenzyme patterns agreed with the differentiation based on biochemical properties. The catalase activity activity staining method was shown to be a restricted yet reliable assay in the intrageneric but not intraspecies differentiation of yellow-pigmented micrococci.