Protein breakdown of 5 to 7% per hr was found in nitrogen-starved cells of an unclassified prototrophic thermophilic bacillus; a similar protein-breakdown rate (6.5% per hr) was found in resting cells of Escherichia coli. In the thermophile, the rate of protein breakdown was markedly influenced by the temperature; it was maximal between 45 and 55 C, and it decreased considerably at 35 and 75 C, temperatures which are only slightly below or above the minimal and maximal growth temperatures. Growing cultures of the thermophile showed little, if any, protein breakdown, a finding similar to that of others with E. coli.