In order to examine temporal changes in germination ability, time to germination and auto-fluorescence property of the resting cysts of Alexandrium tamarense, long-term investigation was conducted in Hiroshima Bay, where spring bloom (March to May) of A. tamarense has been observed almost every year since 1992. Approximately fifty cysts were isolated monthly from the bottom sediment between June 1994 and June 1997. The cysts were incubated on the day of sampling under in situ bottom water temperature condition, and germination success and emission of auto-fluorescence were checked every day. High germination success rates (> 50 %) were observed between December and April every year (bottom water temperature = 10.0 to 16.5 C, with an average germination time of 10.2 days (n = 455). On these occasions, cysts started to emit red auto-fluorescence few days before the germination and no obvious temporal change in germination time was observed. From June to November, germination success rates were considerably low (0 to 40 %, bottom water temperature = 14.6 to 25.1 C). Particularly, no germination was observed in September (bottom temperature = 23.6 to 25.1 C). Relationship between the incubation temperature and the rate of germination success suggested that the cysts have a "temperature window" (ca. 10 to 15 C) for germination, which would be responsible for the seasonal changes in germination ability. The present results indicate that the germination characteristics of A. tamarense cysts are well adapted to the ambient water temperature rhythm in the temperate shallow coastal environment, allowing A. tamarense to seed vegetative cell population for the spring bloom.