Microzooplankton populations and other seston components in the water column were sampled from a central station in Akkeshi Bay, Japan for a year. The measurements reported in the present study constitute the first work on seasonal variation of microzooplankton and other seston components in Akkeshi Bay. Twenty-one categories were identified and their volume measured. T he total volume was highest between February and April, and suddenly decreased in May and then gradually increased during summer. It was relatively low and stable during the fall diatom bloom. and was least between November and January. Abundance of microzooplankton was dependent upon concentration of chlorophyll and could be predicted by% MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaciiBaiaac+% gacaGGNbGaaeywaiabg2da9iaaigdacaGGUaGaaGimaiaaicdacqGH% RaWkcaaIWaGaaiOlaiaaiwdacaaI3aGaciiBaiaac+gacaGGNbGaae% iwaaaa!44F9!\[\log {\text{Y}} = 1.00 + 0.57\log {\text{X}}\]where Y was the volume of microzooplankton and X was the concentration of chlorophyll a. The ecological significance of microzooplankton and other seston components is discussed.