Summary Comparison of relative abundance of each two successive life history stages was made for studying population dynamics of ayu, Plecoglossus altivelis , in Lake Biwa. Number of eggs spawned is proportional primarily to the size of biomass of spawner population although spawning success plainly depends on presence or absence of heavy rain in the spawning period. Although density of fry tends to be proportional to that of egg, yearly fluctuation of the fry mortality may be caused by changes in density of food organisms but is not in compensatory way. From the fry stage to the juvenile one the rate of mortality and/or of growth seem to fluctuate in compensatory way which is supposed to be related to a shortage of food. Yearly fluctuation of mortality of adult stage is caused by an artificial factor as changes of fishing intensity. Necessity of studying actual events at each stage of the life history process separately in relation with environmental conditions is discussed through the result of the present analysis.