Cytokinesis (septation) in the fungus Aspergillus nidulans occurs through the formation of a transient actin ring at the incipient division site. Temperature-sensitive mutations in the sepA gene prevent septation and cause defects in the maintenance of cellular polarity, without affecting growth and nuclear division. The sepA gene encodes a member of the growing family of FH1/2 proteins, which appear to have roles in morphogenesis and cytokinesis in organisms such as yeast and Drosophila. Results from temperature shift and immunofluorescence microscopy experiments strongly suggest that sepA function requires a preceding mitosis and that sepA acts prior to actin ring formation. Deletion mutants of sepA exhibit temperature-sensitive growth and severe delays in septation at the permissive temperature, indicating that expression of another gene may compensate for the loss of sepA. Conidiophores formed by sepA mutants exhibit abnormal branching of the stalk and vesicle. These results suggest that sepA interacts with the actin cytoskeleton to promote formation of the actin ring during cytokinesis and that sepA is also required for maintenance of cellular polarity during hyphal growth and asexual morphogenesis.