The temperature sensitive (ts-) mutants of S. pombe defective in cell cycle progression were isolated by EMS mutagenesis. By using one of mutants, cyj92, a novel gene, $psp1^+$, which functionally complements this ts- phenotype defective in cell cycle progression both in $G_1/S$ and $G_2/M$, has been isolated from the genomic and cDNA libraries of S. pombe. Disruption of this gene is lethal for cell growth at 30℃ indicating that it is an essential gene for vegetative cell growth. Western analysis of the protein by polyclonal antibody made from GST-Psp1 fusion protein indicated that the Psp1 protein exists in two different molecular weight forms depending on the growth state of the cell. In vitro experiments with a phosphatase showed that this difference is due to phosphorylation. The dephosphorylated form of the protein is dominant in actively growing cells whereas the phosphorylated form becomes the major species when cell enters a stationary phase. The Cdc2/Cdc13 complex is shown to phosphorylate the GST-Psp1 fusion protein in vitro and site directed mutagenesis and phosphoamino acid analysis indicated that the serine residue at position 333 in the C-terminal region is required for phosphorylation. Overproduction of the novel protein Psp1 in actively growing cells inhibit cell growth causing accumulation of DNA (4n or 8n). Thus we speculate that Psp1 can function both at $G_1/S$ and $G_2/M$ phase complementing the defect of the new mutant we have isolated. It is likely that Psp1 is required both for proper DNA replication and for the process of mitosis.