We investigated seasonal variations of population structure and depth distribution ofCalanus sinicus in northern Kagoshima Bay. Animals were abundant during April to June whenchlorophyll a concentrations were high. Young copepodites or adults appeared over the seasons,however, copepodite stage 5 dominated when the population abundance was the lowest duringAugust 2009 and July 2010. We found little indication of seasonal vertical migration since alldevelopmental stages occurred above 50 m and below 100 m throughout the year. RNA:DNAratios revealed no significant decline throughout the year. These results suggest that Calanussinicus reproduces throughout the year and does not experience a deep-water dormancy period innorthern Kagoshima Bay.