A homothallic haploid strain of the fission yeast Schizosaccharomyces pombe initiates sexual reproduction (mating, meiosis and sporulation) in nitrogen-free sporulation medium. Cellular fine structures of eleven sporulation-deficient mutants (spo2, spo3, spo4, spo5, spo6, spo13, spo14, spo15, spo18, spo19 and spo20) of S. pombe in sporulation medium were examined by serial section-electron microscopy. The striking features of these spo mutants were: 1) the disappearance of the spindle pole bodies (SPBs) after the second meiotic division, and 2) the accumulation of unorganized structures. Based on histochemical staining, these structures were presumably unorganized spore wall precursors. In some mutants (spo3, spo5, spo6, spo19 and spo20), diploid zygotes contained four spore-like bodies which had walls similar to complete spore walls but failed to enclose any nuclei. After completion of the second meiotic division the nuclei were abnormally distributed in zygotic diploid cells. In the spo5, spo13, spo14, spo15 and spo19 mutants, the nuclei remained attached to each other. In spo5 and spo19, the inner membrane of the nuclear envelope was separated, but its outer membrane was shared by two sister nuclei. These observations suggest that the spo+ gene products play important roles in spatial and temporal organization of cellular structures during ascospore development.