Two novel casein kinase-1 homologs, cki1+ and cki2+, have been isolated from Schizosaccharomyces pombe and characterized. Both genes reside on chromosome II and encode approximately 50-kDa proteins that are related structurally and enzymatically to the YCK gene products of budding yeast. Subcellular fractionation experiments demonstrate that Cki1 and Cki2 are both cytoplasmic enzymes that do not overlap in subcellular distribution and that probably play distinct roles within the cell. Although gene disruption experiments show that neither cki1+ nor cki2+ is essential for cell viability, overexpression of cki2 leads to a severe growth defect and aberrant morphology. Cells become round or pear shaped and separate poorly following septation. These results suggest that of the four members of the casein kinase-1 family recognized in fission yeast, one member, Cki2, may contribute to the regulation of cell morphology.