Of 14 potential sulfur-oxidizing strains, Pseudomonas sp. B21 and Agrobacterium sp. B19 were considered as denitrifiers. Under aerobic conditions, with S-0 as electron donor, maximum cell growth rates were 0.022 (B21) and 0.043 h(-1) (B19). Both grew optimally at pH 7.5 and 28degreesC. When NO3-N was increased from 10 to 200 mg l(-1) the efficiency of nitrate removal of each strain gradually decreased, from 60 to 40%. Addition of suitable organic compounds (C/N<4.2) increased the nitrate removal efficiencies of both strains, indicating their mixotrophic characters.