A temperature-sensitive mutant of a thermophilic bacillus was isolated which was unable to maintain membrane integrity at high temperature. The mutant appeared to lose cytoplasmic contents, as indicated by a decrease in turbidity and cell refractivity, when shifted from a permissive (52 ~ C) to a restrictive (65 ~ C) temperature. Cell number remained fairly constant, however. At the approximate onset of the decline in turbidity, viability decreased and net synthesis of ribonucleic acid, deoxyribonucleic acid, and protein ceased. Both chloramphenicol and sucrose were effective in retarding the decline in turbidity at 65~ An abnormal fatty acid composition at high temperature suggested that the lesion in the mutant involved lipid synthesis. The proportion of fatty acids with a high melting point (> 55 ~ C) increased in the parent from 42 ~ at 42~ to 69 ~ at 65 ~ C. Similar changes were not made by the mutant. An abnormal phospholipid composition was also observed in the mutant at 42~ and 52 ~ C. However, at 58 ~ C, the maximum growth temperature of the mutant, the proportion of major phospholipids (phosphatidylglycerol, phosphatidylethanolamine, and cardiolipin) was similar to the parent strain. The mutant's apparent loss of membrane stability at high temperature and its inability to regulate fatty acid and phospholipid composition in a normal manner suggested that (i) the temperature-sensitivity of the mutant may be a result of a defect in normal lipid metabolism at high temperature and (ii) the normal changes in fatty acid composition observed at increased growth temperatures may be an essential feature of thermophily.