The gene encoding ammonia monooxygenase subunit A (AmoA) was found in three copies of the genome of the chemolithotrophic soil bacterium, Nitrosospira sp. NpAV. The open reading frame and flanking regions of the three copies were isolated from digested size fractionated genomic DNA using oligodeoxyribonucleotide primers and polymerase chain reaction. The three gene copies of amoA were sequenced and the sequences compared to each other. The open reading frames and the upstream and downstream flanking regions were nearly identical in the three copies. All three copies were expressed in recombinant Escherichia coli strains from the indigenous promoter producing a product of approximately 30 kDa. All amoA copies encode 274 amino acid polypeptides which have similarity to the ammonia monooxygenase acetylene-binding protein from Nitrosomonas europaea.