Environmental signals can be transduced into intracellular responses by the action of MAP kinase cascades. Sequential phosphorylation results in the transient activation of a MAP kinase, which in turn activates certain transcription factors and thus a set of pathway-specific genes. Many steps in this cascade are conserved, and homologues have been discovered from yeast to human. We have characterized the MAPKK kinase, SteC, a homologue of Saccharomyces cerevisiae Ste11, in the filamentous fungus Aspergillus nidulans. The 886-amino-acid-long protein shares the highest similarity to Neurospora crassa Nrc-1. Deletion of the gene in A. nidulans results in a slower growth rate, the formation of more branched hyphae, altered conidiophore morphology, an inhibition of heterokaryon formation and a block of cleistothecium development. The gene is transcriptionally activated during asexual development and controls the phosphorylation of two putative MAP kinases.