A comparative study of ultrastructural changes of cells and organellae was conducted on diploid and triploid abalones, Haliotis discus hannai, during their spermatogenesis. The results showed that spermatogenesis in diploid abalone co uld be divided into five stages: spermatogonium, primary spermato cyte, secondary spermatocyte, spermatid and spermatozoon. A series of changes o ccurred in morphology and structure, including chromatin condensation, development and fusion of mito chondria, acrosome formation and cytoplasmic diminution. Compared with diploid abalones, the size of the cells and nucleus in the triploid abalone was significantly increased in all levels of spermatogenic cells. Similar spermato gonia structure was found between triploid and diploid abalones. Except for lysosomes, the number of mitochondria, endoplasmic reticulum and other organellae in the primary and secondary spermatocytes was decreased in triploid abalone. No significant difference was found in the size of mitochondria between diploid and triploid spermatocytes. However, the latter did not have a typical shape and lacked developed cristae. The spermato genesis of triploid abalone was retarded at the spermatide stage. Various abnormal formations and collapsed cells w ere observed, and no sperm was detected.