Large numbers of chromogenic variants were isolated from cultures of a parent strain of Staphylococcus aureus growing in the dialysate but not in the residue of brain heart infusion (Difco). Gas–liquid chromatographic analysis of the dialysate detected 18 amino acids in this medium. Large numbers of chromogenic variants also were isolated from 13 of 18 synthetic media deficient in a single amino acid but not in the complete synthetic medium containing all 18 amino acids. Gas–liquid chromatographic analysis detected marked quantitative differences in the amino acid metabolites present in a complete synthetic medium and the synthetic medium deficient in arginine after growth for 12 days. The data suggest that differences in the amino acid metabolism of the parent and chromogenic variants could account for the population changes observed in brain heart infusion.