Mutation in the bimD gene of Aspergillus nidulans results in a mitotic block in anaphase characterized by a defective mitosis. Mutation in bimD also confers, at temperatures permissive for the mitotic arrest phenotype, an increased sensitivity to DNA damaging agents, including methyl methanesulfonate and ultraviolet light. In order to better understand the relationship between DNA damage and mitotic progression, we cloned the bimD gene from Aspergillus. A cosmid containing the bimD gene was identified among pools of cosmids by cotransformation with the nutritional selective pyrG gene of a strain carrying the recessive, temperature-sensitive lethal bimD6 mutation. The bimD gene encodes a predicted polypeptide of 166,000 daltons in mass and contains amino acid sequence motifs similar to those found in some DNA-binding transcription factors. These sequences include a basic domain followed by a leucine zipper, which together are called a bZIP motif, and a carboxyl-terminal domain enriched in acidic amino acids. Overexpression of the wild-type bimD protein resulted in an arrest of the nuclear division cycle that was reversible and determined to be in either the G1 or S phase of the cell cycle. Our data suggest that bimD may play an essential regulatory role relating to DNA metabolism which is required for a successful mitosis.