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Two groups of bacteria were determined as oligotrophs and eutrophs. The former can grow only on a poor medium and not on a rich medium, and the latter can grow on a rich medium. Based on such trophic nature of bacterial groups, a method was established for counting viable cell number of oligotrophs and eutrophs, employing poor and rich media. The present method was used to examine water samples of fresh and sea water areas including rivers and lakes in Toyama Prefecture, off the coast of Toyama Bay, and open sea area at Yamato Bank in the Japan Sea. Physical and chemical environmental factors in these areas were also estimated. Based on the result of an extensive survey on these oligotrophic and eutrophic water samples, two general conclusions were deduced: (a) Oligotrophs and eutrophs responded quite similarly to respective chemical environmental factors, and (b) the number of these trophic groups remained almost constant at even higher concentrations of these factors. In fresh water areas, the number of oligotrophs usually exceeded that of eutrophs, and limiting substances for the size of these bacterial populations were considered to be nitrate-nitrogen and dissolved organic nitrogen. From the effect of rainfall on the number and trophic flora of bacteria in a river, the source of river bacteria was suggested at least in part to be attributed to the soil bacteria.In conventional studies on emphasis has been laid on some microorganisms in a natural water system, extreme living conditions specific to water an en-

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