The organization of the actin cytoskeleton plays an integral role in cell morphogenesis of all eukaryotes. We have isolated a temperature-sensitive mutant in Schizosaccharomyces pombe, wat1-1, in which acting patches are delocalized, resulting in an elliptically shaped cell phenotype. Molecular cloning and DNA sequencing of wat1+ showed that the gene encodes a 314 residue protein containing WD-40 repeats. Cells lacking wat1+ are slow growing but viable at 25 degrees C and temperature-sensitive for growth above 33 degrees C. At restrictive temperature, wat1-d strains are phenotypically indistinguishable from wat1-1. When combined with a deletion for the wat1+ gene, cdc mutants failed to elongate at restrictive temperature and exhibited alterations in actin patch localization. This analysis suggests that wat1+ is required directly or indirectly for polarized cell growth in S. pombe. Wat1p and a functional, epitope-tagged, version of Wat1p can be overproduced without inducing alterations in cell morphology.