A calcineurin (type 2B)-like protein phosphatase gene designated ppb1+ was isolated from the fission yeast Schizosaccharomyces pombe. The predicted amino acid sequence was 57% identical to rat PP2B alpha. ppb1 null mutant could form colonies at 33 degrees C but the size of the colonies was small at 22 degrees C. Cytokinesis was greatly delayed at 22 degrees C, and a large number of multi-septate cells were produced. The cell polarity control was impaired, causing branched cells. ppb1 null was virtually sterile. These phenotypes were rescued by a plasmid carrying the ppb1+ gene. Multi-septate cells were also produced in wild type at 22 degrees C by cyclosporin A, an inhibitor of calcineurin. This drug effect was enhanced in stst1 null mutant, which was hypersensitive to various drugs and cations. ppb1 null was not affected by cyclosporin A, consistent with the hypothesis that ppb1 is its target. Double-mutant analysis indicated that ppb1 had a function related to that of two other phosphatases, type 1-like dis2 and 2A-like ppa2.ppb1 null-sts1 null showed the severe multi-septate phenotype in the absence of cyclosporin A. ppb1+ and sts1+ gene functions are related. The double mutant ppb1-sts5 was lethal, indicating that the ppb1+ gene shared an essential function with the sts5+ gene. Overexpression of ppb1+ caused anomalies in cell and nuclear shape, microtubule arrays and spindle pole body positioning in interphase cells. Thus the ppb1+ gene appears to be involved in cytokinesis, mating, transport, nuclear and spindle pole body positioning, and cell shape.