In order to establish a large-scale hatchery technique for the sea squirt Halocynthia ritteri, the in£uences of water temperature on spawning induction, larval development and survival of H. ritteri were studied under laboratory conditions. The larvae of H. ritteri exhibited high sensitivity to changes in temperature. As temperature increased from10 to 21 1C, the duration of larval stage was inversely related to temperature and the time to reach attached larvae stage decreased from 42.0 to 59.4 h. The biological minimum temperature for the early development of H. ritteri was estimated to be 0.22 1C. In relation to temperature, quadratic equations showed signi¢cant ¢ts to the data of spawning rate, fecundity, fertilization, larval survival and attachment. Optimal temperatures for spawning rate, fecundity, fertilization rate, larval survival rate and attachment rate were estimated to be 14.9, 14.7, 13.2, 14.1 and 14.7 1C respectively. It is concluded that the range of 13^15 1C is optimal for spawning and larval rearing of sea squirts. Aquaculture Research, 2009, 40, 513^518 In£uence of water temperature on sea squirt K H Kang et al.