A novel sporulation-deficient mutant, sev4-L5, was isolated in a genetic screen of a collection of temperature-sensitive mutants of Schizosaccharomyces pombe. The wild-type sev4 gene was identified as cta4⁺, which encodes a putative cation-transporting P-type ATPase. The sev4-L5 allele harbored a single missense mutation that caused replacement of Gly615 with a glutamate at the putative ATP-binding site. Similar to cta4-null mutants, sev4-L5 exhibited defects in growth at high and low temperatures, and sensitivity to high and extremely low concentrations of Ca²⁺. The cta4⁺ mRNA level was considerably enhanced during meiosis. When sev4-L5 cells were incubated in sporulation medium at the permissive temperature, meiotic nuclear divisions proceeded with normal kinetics, but spores were not formed. Structural alteration of the spindle pole body, which is prerequisite to construction of the forespore membrane in wild type, was incomplete. Consequently, formation of the forespore membrane was severely impaired. These observations show that perturbation of Ca²⁺ homeostasis by mutation of cta4/sev4 blocks sporulation mainly by interfering with forespore membrane assembly.