The gene organization and nucleotide sequence of the type A and B BoNT-gene clusters in Clostridium botulinum strain NCTC 2916 were studied. The aim was to clarify the organization of genes within C. botulinum type A strains possessing an unexpressed BoNT/B gene. The BoNT/A-gene cluster includes genes encoding BoNT, NTNH and a part of P-47 (the gene for this protein was reported in strains of C. botulinum types E and F). Clustered with the silent BoNT/B gene were genes encoding NTNH, P-21 and HA-33. Sequencing analysis of the: NTNHs revealed the presence of 471 amino acids identical in the type B and A gene clusters. This gene organization contrasts markedly with the purported organization in strain NCTC 2916 described by Henderson et al. (FEMS Microbiol. Lett. 140. 151-158). In type A(B) strain NCTC 2916, the neurotoxin gene is of type BoNT/A1 within a gene cluster that has identical organization to that found in BoNT/A2 type strains; these observations may be significant in establishing the origin of the BoNT-gene cluster.