Minimal chemically defined media for Bacillus stearothermophilus were developed at 60°C and quantitative requirements for each nutrient were determined. A prototrophic strain of B. stearothermophilus was grown in medium containing only glucose and mineral salts whereas auxotrophic strains in addition required biotin, thiamine, nicotinic acid and DL‐methionine. Metabolic interaction between L‐valine and L‐leucine was observed with auxotrophic organisms. The presence of L‐leucine in minimal medium necessitated the addition of L‐valine. Growth took place in the absence of both amino acids.