THE synthesis of cysteine in micro-organisms from inorganic sulphur sources has been studied for some time, and the available evidence suggests that sulphate sulphur is reduced to the thiosulphate or sulphide state prior to its introduction into the carbon chain of cysteine1–3, although some investigators have postulated that the formation of sulphur–carbon linkage occurs at the sulphite stage4. Working with mutant strains of Aspergillus nidulans, Hockenhull2 suggested that thiosulphate is the form in which sulphate sulphur combines with the carbon skeleton. Based on the observations that cysteine-S-sulphonate supported good growth of all the thiosulphate-requiring mutants tested and that growth of these strains on thiosulphate was enhanced by the addition of serine, he concluded that thiosulphate first reacts with serine to form cysteine-S-sulphonate as an intermediate and cysteine is formed from the latter by hydrolysis. The same possibility was also postulated by Shepherd4, although the involvement of thiosulphate in the main route of sulphate assimilation was denied by this investigator. Schlossmann and Lynen5, on the other hand, reported the occurrence in yeast of a pyridoxal phosphate-dependent enzyme catalysing the condensation of hydrogen sulphide with serine to form cysteine. The present communication describes more convincing evidence in favour of the cysteine-S-sulphonate pathway in A. nidulans.