In sucrose-limited culture of Penicillium chrysogenum the growth yields, corrected for maintenance, for sucrose and oxygen, at dilution rates below 0.05 h−1, were 86 g dry biomass/mol hexose and 39 g dry biomass/mol O2. Similar yields have been observed with other moulds and with yeasts. In oxygen-limited culture the yield from oxygen was two to three times higher and the yield from sucrose about 10% higher than in sucrose-limited culture. In phosphate-limited cultures the respiration rate was unaffected by oxygen concentrations between 0.06 and 0.18 bar and by the concentration of sucrose in the medium between 0.1 M and 0.3 M. The excess of sucrose over the growth requirement was metabolised to organic acids, predominantly malate and gluconate. The results suggest that, in the presence of excess oxygen, the respiration rate is controlled by the biosynthetic demand for energy.