The nucleotide sequence of the Bacteroides fragilis neuraminidase-encoding gene (nanH) and its flanking regions was determined. The B. fragilis nanH open reading frame (ORF) of 1632 nucleotides encoded a neuroaminidase of 544 amino acid residues with a calculated molecular mass of 59.51 kDa. The amino acid sequence at the N-terminus of the deduced protein exhibited the common features of procaryotic signal sequences. The Asp box sequence, Ser-X-Asp-X-Gly-X-Thr-Trp, which is conserved among bacterial neuraminidases, was found at five positions in the deduced amino acid sequence. A single transcription start point was identified 88 nucleotides upstream of the nanH ORF. Northern blot analysis using an internal nanH probe revealed a single mRNA species with a length of 1.8 kb that could encode just the neuraminidase.