The results of 32 batch tests demonstrate a significant difference in biomass yield under anoxic and aerobic conditions using a variety of soluble substrates. An anoxic yield of 0.402 mg particulate COD/mg consumed COD was calculated which is 62% of the corresponding aerobic yield of 0.645 mg particulate COD/mg consumed COD. Batch tests were performed under anoxic and aerobic conditions using a seed of either activated sludge from one of two local municipal wastewater treatment facilities or a prepared sample of Pseudomonas denitrificans. Irrespective of the test organism, the calculated yields were consistent for each electron acceptor. A novel approach to quantifying the consumption of electron acceptor was employed in these batch tests. COD balance calculations were performed and confirmed that all of the initial COD could be accounted for in terms of residual soluble COD, biomass production and electron acceptor consumption.