Flow microcalorimetry has been used to measure the anaerobic and aerobic rate of glucose catabolism by washed cell suspensions of Streptococcus agalactiae under flow and stopped flow conditions. The rate of glucose catabolism under aerobic and anaerobic conditions was similar, while heat production was greater under aerobic conditions due to divergence from a homolactic fermentation which was confirmed by enthalpy calculations. The pH of the buffer, in the range of 5.5 to 6.8, did not affect the rate of glucose catabolism or heat production. In the presence of 2, 4-dinitrophenol at a concentration optimized for maximum stimulation of glucose catabolism, the increased amount of glucose catabolized appeared as lactic acid under anaerobic and aerobic conditions; buffer pH determined the rate of glucose catabolism, with the maximum rate occurring at pH 6.0. The significance of pH in restricting the growth of different strains of lactic acid bacteria is considered in terms of the provision of energy from carbohydrate catabolism.