One of the defining characteristics of the catalytic subunit of the cyclin-dependent protein kinases (cdks) is the so-called PSTAIRE motif. Western blots of fission yeast cytosolic extracts using a monoclonal antibody against the PSTAIRE peptide revealed two bands at 34 kDa (p34cdc2) and 31 kDa (p31). Polyclonal antibodies to the C-terminus of p34cdc2 or to the full-length protein recognized the 34 kDa band but not p31. Overexpression of the cdc2+ gene resulted in the increase of the 34 kDa band but not p31. Like p34cdc2, the level of p31 revealed no obvious cell cycle regulation but the protein was present in spores where p34cdc2 was barely detectable. p31 expression was unaffected by removal of either phosphate or ammonium from the growth medium, although the level of p34cdc2 was reduced in the absence of phosphate. p31 was not associated with cyclin B, nor was it adsorbed to p13suc1 Sepharose beads, two characteristics of p34cdc2. p31 did, however, interact with p15, the starfish homologue of p13suc1. p31 was present in cells in which cdc2+ was replaced by its budding yeast homologue CDC28. When fission yeast cytosolic extracts were subjected to gel filtration chromatography, p31 eluted in two peaks, one at approximately 100 kDa, the other at approximately 30 kDa. We conclude that p31 is a novel fission yeast PSTAIRE protein and therefore, potentially, a new cdk.