MCM proteins are required for the proper regulation of DNA replication. There are six MCM proteins in all eukaryotes which interact to form a large complex. We report the cloning of fission yeast mcm3 +. mcm3 + is essential and spores carrying a Delta mcm3 disruption arrest with an apparently replicated DNA content. The protein is found constitutively in the nucleus and levels remain constant throughout the cell cycle. Mcm3p binds particularly tightly to Nda4p (Mcm5p), but is loosely associated with the other Schizosaccharomyces pombe MCM proteins. Thus, Mcm3p is a peripheral MCM subunit.