The life history and reproduction of the amphipod Jassa slatteryi were investigated monthly for one year on a seagrass bed (Zostera marina L.) in Gwangyang Bay, southern Korea. The standing crop of seagrass showed two peaks in spring and fall. Population dynamics of J. slatteryi was closely correlated with the standing crop of seagrass, suggesting that there is biological interaction between these two species. The life-history pattern was annual and iteroparous with two principal periods of juvenile recruitment during the year; in spring (March to May) and fall (October to December). Overall sex ratio was female-biased. The mean body length of adults and brood size was larger in the spring breeding period than in the fall. There was a positive relationship between the brood size and body length of ovigerous females. The mean egg volume was more than 0.03 mm³ in fall breeding periods, but in other periods it was less than 0.03 mm³. Egg volume was positively related to body length of ovigerous females. At any given female size, egg volume was significantly higher in the fall than in the spring, whereas brood size in the spring was significantly larger than that in the fall. The reproductive output of females was much greater in the fall than that in the spring, suggesting higher reproductive effort during the fall breeding period.