The fission yeast dis2+ gene encodes one of the two type 1 protein phosphatases (PP1) in this organism. Its semidominant mutant dis2-11 is defective in mitosis. Here we report the characterization of a high dosage suppressor, sds22+, that complements dis2-11. Sequencing of the cloned sds22+ gene predicts a novel 30 kd protein, which consists almost entirely of leucine-rich 22 amino acid repeats and is enriched in the insoluble nuclear fraction. sds22+ is an essential gene required for the mitotic metaphase/anaphase transition; gene disruption causes cell cycle arrest at midmitosis. Unexpectedly, the sds22+ gene becomes dispensable upon high dosage of the PP1 genes. The sds22+ product appears to facilitate PP1-dependent dephosphorylation, but does not substitute PP1. We propose that the sds22+ protein forms a repeating helical rod that is capable of enhancing a PP1-dependent dephosphorylation activity that is essential in midmitosis.