Acetylene reducing (N2-fixing) Entero-bacteriaceae have been isolated from activated sludge plants treating waste from the paper and food industries (103 to 106 cells per ml) and from composting plants handling forest waste (105 to 106 cells per g wet weight). Detailed studies on se-lected strains of all taxa showed that: (1) pure cul-tures were able to utilize a range of carbohy-drates, polyols, amino acids and carboxylic acids as sole sources of carbon (2) high levels of nitro-genase were attained during growth with a range of carbon substrates: highest levels (12—66 n mole C2H4.min−1.mg protein−1) were found for glucose and sucrose, variable levels for polyols, and lower levels for citrate and fumarate (1—23 n mole C2H4.min−1.mg protein−1) (3) organic ni-trogen compounds which were utilized as sole sources of nitrogen did not generally repress the synthesis of nitrogenase, although low levels were found for some strains during growth with glu-cosamine. Samples from a laboratory model acti-vated sludge system showed a mean rate of acety-lene reduction corresponding to the fixation of 26 μg N.h−1.1−1, and direct analysis of the in-fluent and effluent waters and sludge showed a net increase in nitrogen. These observations corre-lated with the presence of a population of N2-fix-ing Enterobacteriaceae of ca. 105 cells per ml and pure strains isolated from the system had a mean nitrogenase specific activity of 88 n mole C2H4.min−1.mg protein−1. It is therefore con-cluded that endogenous N2-fixing Enterobacteria-ceae contained in some kinds of industrial waste-waters could successfully be used to diminish the addition of combined nitrogen to activated sludge treatment plants.