From cell yields of Thiomicrospira denitrificans grown inthe chemostat at different growth rates under anaerobic conditions a value of 1.4 mM S2O3 = per g dry wt and per h could be calculated for maintenance energy requirements, and of 5.65 dry wt per mole S2O3 = for the true growth yield. Cell yields of Thiomicrospira denitrificans appeared to be almost half of those of Thiobacillus denitrificans. Though in Thiobacillus denitrificans at D = 0.03 h(-1) under anaerobic conditions a value was found of 11.60 g dry wt per mole of thiosulphate used for energetic purposes, a value of 5.72 g dry wt per mole of thiosulphate was found under comparable conditions in Thiomicrospira denitrificans. Under aerobic conditions at D = 0.03 h(-1) values of 18.54 g dry wt per mole of thiosulphate were found in Thiobacillus denitrificans whereas Thiomicrospira denitrificans yielded only 9.38 g dry wt per mole of thiosulphate. As in Thiobacillus denitrificans anaerobic cell yields on sulphide were comparable to those on thiosulphate. Calculations have been made which indicate that the biosynthetic efficiency of Thiomicrospira denitrificans is lower than that of Thiobacillus denitrificans. This can only partly be explained by the absence of adenosine-phosphosulphate (APS) reductase.