ABSTRACTA sporulation-deficient (sporeless) mutant inLentinus edodesgave rise to basidiocarps visibly indistinguishable from those of wild-type dikaryons. In the basidia, karyogamy and meiosis occurred normally and sterigmata developed after meiosis, but the distal ends of the sterigmata usually did not swell. Up to the stage of sterigmal development, the nuclei, mitochondria and other cell organelles in the basidia appeared to be intact. Basidia bearing only two or three sterigmata occunred more often in the mutant than in the wild type. Basidia with eight nuclei were not found in the mutant, indicating that post-meiotic mitosis did not occur. These results suggested that, in most cases, the process of sporulation in the mutant was arrested during or right after the development of sterigmata. Genetic analysis of this mutant indicated that the sporulation deficiency was controlled by a single recessive gene,spo, which was not linked to either theAor theBincompatibility factor.