A bacterial contaminant from a cellulase-producing culture ofTrichurus cylindricuswas subjected to physiological and biochemical tests, and identified as a member of the genusMicrococcus, subgroup 6 (Baird-Parker). It appeared either to stimulate cellulase production by the fungus, or to increase cellulase activity. The amino acid and protein content of theMicrococcussuggested that it might be useful as a feed supplement. In a preliminary trial, rats accepted freeze-dried cells as a partial source of energy, and grew at a normal rate. No toxic effects of the diet were noted.