The present work is concerned with the mechanism involved in natural gynogenesis in crucian carp, Carassius auratus gibelio. (1) Fertilization Characteristics of the Sperm of Gynogenetic Crucian Carp (Carassius auratus gibelio) in the Egg of Amphimictic Fish Cytological study on the fertilization with sperms of gynogenetic crucian carp and eggs of amphimictic common carp has been made in order to reveal the mechanism which leads to the death of the hybrid embryos. It showed that the sperms were able to transform into male pronuclei and fuse with female pronuclei, so that the zygotic nuclei were formed. However, after the beginning of cleavage, some unusual phenomena, such as the elimination of chromosomes, the formation of multiple spindles and the asynchronous division of the two blastomeres of the same egg, were observed. From these results, the authers are preliminarily of the opinion that there exists a certain chromosome incompatibility between gynogenetic crucian carp and other fishes, which leads to the repelling of chromosomes in the hybrid embryonic cells. Probabily this reflects the unique genetic background of the chromosome complement of gynogonetic crucian carp. (2) A Preliminary Study on the Mode in the Egg of the Naturally Gynogenetic Crucian carp for Inhibiting Sperm from Transforming into Male Pronucleus The egg of naturally gynogenetic crucian carp can ine hibit sperm (heterologous or homologous) from transforming into male pronucleus so that ontogenesis is completely controlled by the female nucleus. By removing female nucleus, transferring egg cytoplasm and inseminating the egg after the removal of chorion, the present experiment preliminarily revaled the way the egg inhibits sperm nucleus from transforming into pronucleus. On the basis of the experimental results, a hypothesis of double control is advanced: the gynogenetic egg has primary control and secondary control over the sperm. The primary control may be accomplished by a certain special inhibitor located in the passage where the sperm enters the egg. The secondary control may be due to the lack of an unknown substance generally existing in the egg cytoplasm of amphimictic fish to promote the formation of male pronucleus. (3) Development of Sperm Nucleus of Artificially Sexreversed Gynogenetic Crucian Carp in Amphimictic and Gynogenetic Egg Our experimental results showed that the sperm of functional male gynogenetic crucian carp induced by the administration of methyltestosterone to young genetic Iemale could fertilize the egg of amphimictic common carp and transform into male promucleus normally. But after the beginning of cleavage some unusual phenomena were observed such as elimination of nuclear material, multipolar division etc., which convinced us of the hypothesis of chromosome incompatibility advanced in the first part. Beyond expectation, the sperm nucleus of sex-reversed gynogenetic crucian carp could partially transform into male pronucleus in the egg of gynogenetic crucian carp, but its development was slower than female nucleus. According to the hypothesis of double control, we are preliminarily of the opinion that the sperm of sexreversed gynogenetic crucian carp was not inhibited by primary control when it entered the gynogenetic egg, which was perhaps due to the lack of a certain specific receptor of sperm caused by genetically female basis. Because of the presence of secondary control in the gynogenetic egg cytoplasm, the decondensed sperm nucleus could not transform into malo pronucleus completely.