The ammonia-oxidizing chemoautotrophic Nitrosomonas sp. strain K1 exhibited marked ribulose-1,5-bisphosphate carboxylase (RubisCO) activity. The RubisCO IEC 4.1.1.39] was purified as an electrophoretically homogeneous protein. The molecular mass of the enzyme was estimated to be about 460 kDa by gel filtration, and it consists of two subunits [large (L): 52.2 kDa; small (S): 13.3 kDa] as demonstrated by SDS-PAGE. This confirmed that the enzyme has an L8S8 structure. The K-m values of the enzyme for RuBP, NaHCO3, and Mg2+ were estimated tb be 0.112, 0.415, and 1.063 mM, respectively. The optimum pH and temperature for its activity mere approximately 7.0 and 45 degreesC. The enzyme was stable up to 45 degreesC and in a pH range from 7.0-9.0 (4 degreesC, 48 h). The enzyme activity was inhibited by Cu2+ , Hg2+, N-ethylmaleimide, p-chloromercuribenzoate, and SDS (0.1 mM). The activity was also inhibited by ammonium sulfate at high concentrations (38-303 mM) but the stability of the enzyme showed no inhibition at the same ammonium sulfate concentrations. The N-terminal amino acid sequences of the large and small subunits are AIKTYQAGVKEYRQTYW QPDYVPL and AIQAYHLTKKYETFSYLPQM, respectively.