The ste12+ gene of Schizosaccharomyces pombe codes for a phosphatidylinositol (PI) 3-phosphate 5'-kinase, which is required for efficient mating. Suppressor mutants for sterility of ste12Delta cells were screened for. Most of the mutant genes turned out to be recessive. Six genes were cloned and the open reading frames responsible for the suppressor activity were identified. They included genes coding for proteins with domains homologous to calcium transporters, casein kinase II, UBC13, AMSH, Vps23p, and Vps27p of Saccharomyces cerevisiae. Disruption of these genes resulted in suppression of the defects of the ste12Delta cells, including low mating efficiency and formation of large vacuoles. Since many of these gene products are homologous to the proteins involved in vesicle transport, sterility caused by inactivation of ste12 may be due to a disordered vesicle transport system.