Homocysteine is an intermediate in methionine synthesis in Aspergillus nidulans, but it can also be converted to cysteine by the reverse transsulfuration pathway involving cystathionine beta-synthase (CBS) and cystathionine gamma-lyase (CGL). Because homocysteine is toxic to the cell at high concentrations, this pathway also functions as a means of removal of its excess. We found that the transcription of the mecA and mecB genes encoding CBS and CGL was upregulated by excess of homocysteine as well as by shortage of cysteine. Homocysteine induced transcription of both genes when added to the growth medium or overproduced in a regulatory mutant. The derepressing effect of cysteine shortage was observed in some mutants and in the wild-type strain during sulfur starvation. An increase in the level of mecA or mecB transcript roughly parallel with the elevation of the respective enzyme activity. On the basis of the mode of mecA and mecB regulation by homocysteine, these genes may be classified in a group of genes upregulated directly or indirectly by this amino acid. We call this group of genes the "homocysteine regulon".