We have used degenerate oligonucleotide probes based on sequences conserved among known protein tyrosine phosphatases (PTPases) to identify two Schizosaccharomyces pombe genes encoding PTPases. We previously described the cloning of pyp1+ (S. Ottilie, J. Chernoff, G. Hannig, C. S. Hoffman, and R. L. Erikson, Proc. Natl. Acad. Sci. USA 88:3455-3459, 1991), and here we describe a second gene, called pyp2+. The C terminus of each protein contains sequences conserved in the apparent catalytic domains of all known PTPases. Disruption of pyp2+ results in viable cells, as was the case for pyp1+, whereas disruption of pyp2+ and pyp1+ results in synthetic lethality. Overexpression of either pyp1+ or pyp2+ in wild-type strains leads to a delay in mitosis but is suppressed by a wee1-50 mutation at 35 degrees C or a cdc2-1w mutation. A pyp1 disruption suppresses the temperature-sensitive lethality of a cdc25-22 mutation. Our data suggest that pyp1+ and pyp2+ act as negative regulators of mitosis upstream of the wee1+/mik1+ pathway.