There is increasing evidence that the mechanisms controlling the eukaryotic cell cycle are regulated by protein phosphorylation/dephosphorylation cascades. The catalytic subunit of the protein phosphatase 1 is implicated genetically and biochemically in the complex network that regulates mitosis. To investigate further the cell division in plants, we have isolated and characterized two full-length cDNAs from Arabidopsis thaliana, PP1A-At1 and PP1A-At2, encoding polypeptides highly homologous to known protein phosphatase 1 (PP1). DNA gel blot analysis suggests that the protein phosphatases 1 might form a small gene family in Arabidopsis. Northern analysis shows that transcripts are present in all plant organs. In cell cultures, the PP1 mRNA levels are differentially affected by treatment with drugs that block cell division. The expression of PP1A-At1 in a Schizosaccharomyces pombe cdc25ts/wee1- double-mutant strain restores temperature sensitivity, showing that the Arabidopsis phosphatase gene is capable of interacting with genes that regulate the fission yeast mitotic apparatus. However, the dis2-11 S. pombe strain, which has a cold-sensitive allele of the phosphatase 1 gene, is not rescued by expression of the PP1A-At1 gene, suggesting that the plant cDNA is not a functional homolog of the fission yeast gene.