AbstractSeasonal variation of the biomass of plankton in Lake Yunoko, a eutrophic lake in Japan at an altitude of 1478 in, was followed by measuring simultaneously the quantities of chlorophyll, protein, RNA and DNA in water samples harvested from different depths at a definite site in different seasons of a year. The contents of the four cellular constituents and the sum of them in a given water column showed characteristic seasonal changes during the year. Various ratios derived from the four contents were grouped into two distinct types; one is the winter‐spring type and the other the summer‐autumn type. It was found that the sum of the four contents and the ratio of the protein‐content to the RNA‐content were quantitatively related to the chlorophyll‐content of the water. Based on the assumption that a certain water sample contained only photosynthetic plankton, analyses were made to estimate the proportions of photosynthetic and non‐photosynthetic plankton to the total planktonic mass, the growth rates of these plankton, and the net productivity