Bull. Jpn. Soc. Fish. Oceanogr. 74(sp), Page 58-65, 2010   Environmental regulations of reproductive cycles in teleosts Akio Shimizu National Research Institute of Fisheries Science, Fisheries Research Agency, 2-12-4 Fukuura, Kanazawa, Yokohama, Kanagawa 236-8648, Japan e-mail: aneko@affrc.go.jp Fishes inhabiting the temperate zone show various types of the spawning season. Warm temperature is the important factor which causes initiation of the spawning period in both of “spring spawning” and “spring-to-summer spawning” teleosts, and high temperature during summer is the predominant factor that causes termination of the spawning period in the “spring spawning” species. In the spring-to-summer spawning species, short daylength in autumn is also the predominant factor that causes the termination of the spawning period. In autumn spawning teleosts, short daylength is the predominant factor that causes initiation of the spawning period, and low temperature during winter is the predominant factor that causes the termination. In summer spawning, spring and autumn spawning, and “winter-to-spring” spawning species, changes in water temperature seem to be the important factors for both initiation of and termination of the spawning period. In addition to the environmental factors, internal factor such as circa-annual rhythm may also be concerned with the controlling mechanism of the annual reproductive cycle, through changes in photoperiodism appearance and/or existence of the refractory period. Key words: reproductive cycle, spawning period, photoperiod, temperature