In order to elucidate the mechanisms and degree of isolation in the landlocked form of the threespine stickleback, Gasterosteus aculeatus (L.), inhabiting the Shinano and Agano Rivers that flow into the Japan Sea, comparative studies on the morphology, behavior and histology were made of the marine form, the landlocked form and their hybrids reared in the laboratory. The body shape, coloration and behavior of reciprocal hybrids were almost intermediate between the two parent forms. From January to February, the testis of the hybrid between the female marine form and the male landlocked form is occupied exclusively by spermatocytes and some spermatids. However, a small number of large cells, identified as macrophages, are found invading the seminiferous tubules, indicating the occurrence of phagocytosis. On the other hand, ripe functional eggs appear in the ovary in May, i.e. the breeding season. The eggs deposited into the nest do not develop even when the hybrids display the same mating behavior as their parents. Functional sperms are found in the testis of the hybrid between female landlocked and male marine forms at the period of breeding. However, only a single female lived until March. In the ovary of this female, all eggs reaching vitellogenesis (= primary yolk globule stage) are collapsed, while the younger oocytes appear in healthy condition. The hybrid female of this pairing seemed also to be sterile.