The structural gene of the ferredoxin:sulphite reductase (EC 1.8.7.1) from the cyanobacterium Synechococcus PCC7942 (formerly 'Anacystis nidulans') was cloned and sequenced. The gene termed 'sir' was detected by heterologous Southern hybridisation with the structural gene cysI from Escherichia coli encoding the iron-sulphur haemoprotein of the NADPH:sulphite reductase. The open reading frame is comprised of 1875 bp encoding for a polypeptide of M(r) 70.028. The deduced amino acid sequence is 35.6% identical with the enterobacterial iron-sulphur haemoprotein. This putative fd-dependent sulphite reductase is only distantly related to the fd-dependent nitrite reductase (binary matching coefficient SAB: 0.23) or with the NADPH-sulphite reductase (SAB: 0.32). Highly conserved residues are found within the two Cys clusters forming the reactive Fe4S4-sirohaem centre of the enzyme. Expression of the sir gene using a fusion vector gave a single gene product which is immunologically related with the fd-sulphite reductase from the wild-type bacterium.