The Schizosaccharomyces pombe gene pch1(+) (pombe cyclin C homology) was isolated in a two-hybrid screen for proteins that interact with Cdc2. The cyclin box region of Pch1 protein shares greatest sequence identity with mammalian and Drosophila C-type cyclins ( approximately 33% identity). Pch1 is significantly less similar to Mcs2 (19% identity), a second member of the C-type cyclin family in S. pombe. Cdc2 co-precipitates with Pch1 in S. pombe cell lysates, although Cdc2 may not be the major catalytic partner of a Pch1 kinase in vivo. Purified Pch1-associated kinase phosphorylated myelin basic protein, histone H1, and a peptide corresponding to the carboxyl-terminal domain repeat of RNA polymerase II. The amount of pch1 mRNA does not oscillate during the cell cycle, as is the case for mRNA transcripts of other C-type cyclin genes. Deltapch1 cells are inviable, therefore S. pombe has two essential genes that encode members of the C-type cyclin family, pch1(+) and mcs2(+). The Deltapch1 mutation causes pleiotropic morphological defects and an associated growth deficiency, but loss of Pch1 activity does not result in a cdc cell cycle-arrest phenotype.