Like other filamentous fungi, Aspergillus nidulans forms a multitude of cell types that facilitate colonization and development. The molecular basis of cellular morphogenesis in A. nidulans is not well understood.Here, we summarize results obtained from detailed annotation of the A. nidulans genome sequence for genes with predicted roles in morphogenesis, with primary focus on polarized growth, calcium signaling, and development. We draw three broad conclusions from our results. First, the components of the signal transduction pathways and morphogenetic machinery as defined in the model yeasts Saccharomyces cerevisiae and Schizosaccharomyces pombe are largely conserved in A. nidulans. Second,A. nidulans possesses many additional genes implicated in morphogenesis that are not conserved in these yeasts. Third, the number of A. nidulans genes involved in morphogenesis is likely to be rather large;based on our annotation, we estimate that as many as 2000 A. nidulans genes encode proteins that may participate at some level in morphogenesis during vegetative growth and development.