Planktonic crustaceans were collected monthly around Hamduck, northern part of Cheju Island from April 1989 to March 1990. Occurrence of mean individuals (244 rods. m^(-3)) from inner~ay stations was 2.4 times higher than that (102 rods. m^(-3)) in outer-bay stations. Mean biomass (wet weight, 18.6 mgm^(-3)) from inner-bay stations was 3.6 times higher than that(5.2 mgm^(-3)) in outer-bay ones. The estimated mean values (251.5 ㎍Cm^(-3)d^(-1)) of daily production from inner-bay, having 141.1 ㎍Cm^(-3)d^(-1) of annual mean, was 2.8 times higher than that (90 ㎍Cm^(-3)d^(-1)) in outer-bay. The ratio of daily production of biomass with average value 0.17 suggested a high turnover rate to maintain a temperate costal ecosystems, in spite of a low biomass around Cheju Island.