The turnover of protein in a prototrophic strain of Bacillus stearothermophilus during exponential growth in a salts medium with glucose or succinate as carbon source was about 4 %/h and in a richer nutrient broth medium about 23 %/h. Protein degradation under non-growing conditions conformed to a similar pattern. The turnover of RNA (non-messenger) was about 1 %/h in salts medium and about 9 %/h in nutrient broth. The turnover of protein and RNA in the thermophile is thus moderate rather than massive. This conclusion was confirmed by measurement of the decay of a specific enzyme, isocitrate lyase, in the prototroph and of the overall protein turnover in a non-prototrophic strain of B. stearothermophilus. The half-lives of a number of enzyme systems in intact cells of the prototrophic thermophile at its optimum growth temperature showed some variation but indicated a significant rate of inactivation. Such decay of protein in vivo apparently accounts for the moderate protein turnover observed during growth.