The temperature-sensitive cell cycle mutation bimE7 of Aspergillus nidulans causes cells to become blocked in mitosis at a restrictive temperature. Previous work has shown that this mitotic block is induced even when cells are arrested in the S or G2 phase. The mitotic block is also observed in cells carrying a null mutation in bimE, obtained by molecular disruption of the gene (Osmani, S.A., Engle, D.B., Doonan, J.H., and Morris, N.R. (1988) Cell 52, 241-251), indicating that a lack of bimE function is responsible for the phenotype. We have cloned the bimE gene by complementation of the mutant phenotype and have isolated and sequenced its corresponding cDNA. The gene product is encoded by a 6.5-7-kilobase mRNA. The deduced amino acid sequence suggests a protein with three transmembrane domains. The sequence contains numerous potential N-glycosylation sites and several putative cAMP-dependent phosphorylation sites. No homologous protein sequences were found in the common data bases. The bimE gene product is a novel component in the regulation of mitosis.