Plankton samples were collected in 1960～1963 from Po-hai and Yellow Sea for the purpose of studying mainly the marine zooplankton and the feeding habits of economic marine fishes.The individual weight of those species of zoo- plankton and a few species of diatoms which might serve as food for economic fishes had been measured in various seasons and at different stages of develop- ment(or at different lengths).Species more than 0.5 mm.in size were mea- sured by weight.Those less than 0.5 mm.were measured by volume and then the avearage volume multiplied by the specific gravity(1.04).Ten groups of zooplankton were measured,including 27 species,10 kinds of larvae,2 kinds of eggs and 4 species of Coscinodiscus(Tabs.1～5). The results of the individual weight measurements of zooplankton in Po- hai and the Yellow Sea showed that:(1)The female is always larger in size and heavier in weight than the male in all species.(2)There exists a marked seasonal variation in individual weight of zooplankton,i.e.the largest in wei- ght always appeared in winter(Dec.-Feb.),the medium in spring(March- May),and the smallest in summer(June-Aug.)and autumn(Sept.-Nov.).