Aspergillus nidulans reproduces asexually by forming thousands of mitotically derived spores atop highly specialized multicellular organs termed conidiophores. We have identified a gene called flbA (for fluffy low brlA expression) that is required for initiation of A. nidulans conidiophore development. flbA mutants form abnormal colonies that have a distinct fluffy phenotype characterized by tightly interwoven aerial hyphae that autolyse as the colony matures. The requirement for flbA in conidiophore development precedes activation of brlA, a primary regulator of conidiophore development. The wild-type flbA gene was isolated and found to encode a 3.0 kb mRNA that is expressed throughout the A. nidulans asexual life cycle. Overexpression of flbA using an inducible promoter resulted in misscheduled expression of brlA in vegetative cells and caused hyphal tips to differentiate into spore-producing structures. Sequence analysis of a nearly full-length flbA cDNA clone showed that flbA is predicted to encode a 717-amino-acid polypeptide with 30% identity to the Saccharomyces cerevisiae SST2 protein. SST2 is required by yeast cells for resuming growth following prolonged exposure to yeast mating pheromone and for mating partner discrimination. We propose that flbA plays a related role in a signalling pathway for Aspergillus conidiophore development.