In this study, the activities of hydrogen sulphide (H2S) oxidation and nitrate (N-NO3-) reduction by three pure and mixed strains of nitrate-reducing, sulphide oxidizing bacteria (NR-SOB) were determined. Batch experiments were performed at 35 degrees C and pH 7.0-8.0 with initial H2S concentrations of 650-900 ppm(v) and N-NO3- concentrations of similar to 120 mg/L. The strains MAL 1HM19, TPN 1HM1 and TPN 3HM1 were capable of removing 100% gas-phase H2S. The co-cultures showed better performance for H2S and N-NO3- removal. The mixed NR-SOB strains showed a higher H2S oxidation rate (143 +/- 18 ppm(v)/h), while the highest N-NO3- removal rate (5.5 +/- 0 and 5.1 +/- 0.6 N-NO3- mg/L.h) was obtained by a mixture of two NR-SOB strains. The 16S rDNA sequence analysis revealed that all strains belonged to the sub-class Alphaproteobacteria and are closely related to Paracoccus sp. (> 99%).