Chronic action of adverse factors on the body leads to violations of cytogenetic stability and accumulation of chromosomal anomalies in cells of the body. Therefore, it is expedient to use the micronuclear test, which belongs to one of the most effective methods, and allows to determine the action of substances, which occur during the start of biofilter, on the structure of chromosomes and to detect genetic changes in cells of the fish. The purpose of work was to investigate the formation of micronuclei in cells of gills of hatchling of rainbow trout during the launch of biofilter RAS for the use of polypropylene filler RK PLAST in it. As a result of histological evaluation of cells of gills of rainbow trout, three types of micronuclei were identified. The 1st type included cells of gills of hatchling of rainbow trout that contained 1 micronucleus, which was located at a great distance from the main nucleus, the 2nd type included cells of gills that contained 2 micronuclei, which were closer to the periphery of cells, and the 3rd type - cells of gills that contained 3 or more micronuclei. The vast majority of cells of gills of rainbow trout contained 1-2 (80 %), and also cells with 3 micronuclei were encountered
nitrite, rainbow trout, biofilter, micronuclei