The respiratory denitrification activity of activated sludge bacteria with different carbon sources (acetic acid, crude syrup, hydrolyzed starch, methanol-with and without a small amount of yeast extract) was studied in long-term continuous cultivations and batch tests. Mass balance calculations showed that the main product in long-term cultivations with all carbon sources was molecular nitrogen. However, the type of carbon source had a significant influence on the denitrification rate, denitrification yield, sludge yield and the composition of the microflora. With acetate and methanol higher denitrification yields, lower sludge yields and more true (end product N-2) denitrifying bacteria were obtained than with crude syrup and hydrolyzed starch. Furthermore, with acetate a higher growth rate and a higher denitrification rate was obtained than with methanol.