A temperature sensitive mutant of Saccharomyces cerevisiae was isolated with partially defective respiratory activity at high temperature. The respiratory activity of the mutant was reduced to approximately 1/4 of the parental strain after grown at 30°C, and it could be improved to be normal after grown at 20°C. Spectrophotometric analysis showed that the mutant could synthesize only small amounts of cytochromes b and a at 30°C. Genetic analyses revealed that two gene mutations for the respiratory activity of the mutant were involved.