The research covered the determination: of the numbers or hcicrorrophic bacteria: psychrophil ic, psychrctolcrant, mcsophilic and percentage participation or hemolytic bacteria and .ieromonas hydrophila (with acrolysinc and hcmolysine genes) in the waters of the Drwęca River depending on environmental Ilictors and fishery management. The mean quantities 01· hclcrotrophic bacteria (Ht'C) at 4, 14 and 2~°C ranged: O. 78-7.57-101, 1.40-6.65-101 and 1.93-16.23- 103 efuen -3, respectively. The percentage participation 01· hemolytic heterotrophic bacteria (HemPC) and A. hvdrophila among psychrophilic, psychrotolcrant, mcsophilic microorganisms determined at 4, 14, 28°C, ranged: 7.9-10.4, 6.8-12.2, 8.6-22.0 ,111d 1.1-6.4%, respectively. Statistically significant correlation between examined bacteria and temperature values, flows and O2 saturations confirm that the occurrence of those microorganisms depends on the degree of microbiological contamination of that ecosystem, resulting from the fishery management and environmental factors.