Although Propionibacterium acnes ordinarily grows as an anaerobe, this organism was commonly seen on aerobic primary cultures (directly from skin) of the forehead, axilla, deltoid area, and antecubital fossa. On subculture, strains from aerobic primary cultures showed their normal anaerobic state. On repeated tests of the foreheads of 7 subjects over periods of 1 to 4 yr, 24% of all cultures were positive for propionibacteria (P. acnes) on aerobic culture. This phenomenon was equally common in cultures from those with relatively sparse or dense populations of P. acnes on the skin surface. Several different patterns of aerobic growth were observed: satellitism, profuse growth at one dilution and none on the next higher dilution, and proportionate growth in successive dilutions. These different growth patterns indicate that more than one mechanism is operative in stimulating the aerobic growth of propionibacteria. Whether one or more of these mechanisms is operative in vivo, we do not know, but it appears that they do not account for the great individual differences in population density of P. acnes on such sites as the forehead.