The functions of at least four potential regulatory genes are known to overlap temporally during elaboration of the multicellular asexual reproductive apparatus (conidiophore) of Aspergillus nidulans. One of these, the stuA (stunted) gene, has been previously classified as a morphological modifier essential for correct spatial organization of the conidiophore. The gene was cloned by complementation of a strain carrying the stuA1 mutation and has been localized to a 5.0 kb KpnI fragment that encodes a 3.3 kb mRNA. The stuA mRNA was detected at very low levels in mature conidia and in somatic hyphae that had not established developmental competence. A dramatic increase in the abundance of this mRNA occurred coincidentally with the establishment of competence, but prior to the induction of conidiation. RNA abundance remained at this elevated level during conidiophore morphogenesis. These results are consistent with genetic data suggesting that stuA gene function is required from the very earliest events of asexual reproduction until completion of conidiophore development, but is not specifically required for differentiation of conidia. The expression of the stuA transcript was not affected by any of the other characterized developmental mutations. These latter results suggest that transcriptional activation at the onset of competence is mediated by an as yet unidentified genetic locus or loci.