The abundance and the population composition of marine heterotrophic bacteria. in the organically polluted waters off the Haihe River mouth were investigated and studied. It has been found that the density of heterotrophic bacteria on the surface water of the debouch is higher, generally over 105-106 Ind/mL in the estuary affected by tides and averaging to 103 Ind/mL in the beachy sea outside the debouch. The quantitative distribution of terrigenous freshwater bacteria shows remarkable seasonal difference under the direct control of run-offs. Evaluation of 99 strains of heterotrophic bacteria isolated from the beachy sea outside the debouch shows that the number of Gram-positive bacteria is grater than that of the Gram-negative, the dominant species are terrigenous Micrococcus and Bacillus of Gramlpositive bacteria and Alcaligenes of Gram-negative bacteria. From both:the quantitative distribution characteristics of the heterotrophic bacteria and its compositional features, we may well conclude that the heterotrophic bacteria in organically polluted sea area of river mouth are mainly of terrigenous freshowater types.