Аннотация
A total of 56 taxa of zooplankton, 108 taxa of zoobenthos, and 26 species of fishes was found in the middle reaches of the Lena River. Contamination of the river resulted in the decrease in biological diversity, and also in changes in the structure of zoocenoses. Anomalies were registered in the structure of the organs of fishes. In addition, they were characterized by a rather high content of heavy metals. Results of the study of hydrobionts suggest that the middle reaches of the Lena River are assigned to slightly contaminated water courses.