In Salmonella typhimurium and Escherichia coli the biosynthesis of L-cysteine from L-serine and inorganic sulphate proceeds along a branched convergent pathway along one arm of which sulphate is reduced to sulphide, while on the other L-serine is acetylated to O-acetyl-L-serine. This system is subject to positive genetic control in which growth on a poor sulphur source, O-acetyl-L-serine and the product of the cysB regulatory gene are all required for derepression. The final step consists of the formation of L-cysteine from O-acetyl-L-serine and sulphide. We find that in S. typhimurium this reaction is catalysed by two different enzymes, O-acetylserine sulphydrylase A and O-acetylserine sulphydrylase B, coded for by cysK and cysM respectively. Both enzymes are under the control of the cysteine regulon, and either alone is sufficient for cysteine prototrophy during aerobic growth. Although the advantage to the bacterium of having two separate enzymes to carry out the same reaction is unclear, preliminary data suggest that O-acetylserine sulphydrylase B is preferentially utilized for cysteine biosynthesis during anaerobic growth. We speculate that one enzyme may prefer free sulphide as a substrate while the other may use a bound form of sulphide.