Mutations in the Schizosaccharomyces pombe cdc8 gene impair cytokinesis. Here we clone cdc8+ and find that it encodes a novel tropomyosin. Gene disruption results in lethal arrest of the cell cycle, but spore germination, cell growth, DNA replication and mitosis are all unaffected. Haploid cdc8 gene disruptants are rescued by expression of a fibroblast tropomyosin complementary DNA. Immunofluorescence microscopy of wild type and cdc8 gene disruptants indicates that cdc8 tropomyosin is present in two distinct cellular distributions: in dispersed patches, and during cytokinesis as a transient medial band. Collectively these results indicate that cdc8 tropomyosin has a specialized role which, we suggest, is to form part of the F-actin contractile ring at cytokinesis. These results establish the basis for further genetic studies of cytokinesis and of contractile protein function in S. pombe.