We investigated the molecular organization of the region of Aspergillus nidulans chromosome I containing yA, a gene encoding the developmentally regulated enzyme conidial laccase. DNA fragments were identified that complemented the yA2 mutation and were shown to correspond to yA by genetic mapping and gene disruption experiments. The molecular map of the region was oriented to the genetic map by testing DNA fragments for their ability to complement a mutation in the tightly linked adE gene. The yA gene codes for a 2200 nucleotide mRNA that is present at low levels in vegetative cells and mature conidia, but accumulates to high levels in sporulating cultures. yA mRNA appears shortly after differentiation of sporogenous phialide cells. It accumulates in two developmentally abnormal mutant strains that produce phialides but is absent from two mutant strains that do not produce phialides. Thus, yA transcription is probably restricted to phialides. This result is discussed in relationship to the physiological roles played by phialides in spore differentiation.