The organic and inorganic requirements were determined for growth and pigment production by Pseudomonas nigrifaciens, an organism causing a reddish brown discoloration on the surface of mildly salted butter. Amino acids such as alanine, asparagine, proline, or glutamic acid were the best source of nitrogen for both growth and pigment production in a synthetic medium containing sodium chloride, inorganic salts, and glycerol; however, a carbohydrate was required only for pigment production in this amino acid medium. Na, K, Mg, S, P, and Cl or Br were also essential for both growth and pigmentation, larger amounts of these elements usually being required for synthesis of pigment than for growth. The dark brown-purple pigment produced was insoluble in all the common organic solvents hot or cold. With proline as a source of nitrogen a reddish-purple pigment developed which became deep purple then a deep brown purple. It is suggested that two pigments may be formed; one, a melanin-like dark brown substance originating possibly by polymerization of certain oxidation products of tyrosinase, the other, a dark purple condensation product of a secondary amine (such as proline) with the product (such as o-quinone) of a tyrosinase-catalyzed oxidation of phenolic substances produced by the organism.