In 2014 and 2016 in Poland a few disease outbreaks caused by Kocuria rhizophila and Micrococcus luteus were diagnosed in rainbow trout and brown trout. In each of these events, abnormal mortality (approximately 50%) was accompanied by pathological changes in external tissues and internal organs. Exophthalmia, swollen abdomen, increased skin melanisation as well as skin petechiae and focal lesions were often seen in moribund fish. In many fish inflammation of the intestine, liver congestion, and hemorrhages in the tail part of the muscles were also observed. Samples from moribund fish internal organs were inoculated onto blood agar and trypticase soya agar, and incubated for up to 3–4 days at a temperature of 27 °C. In the majority of cases uniform growth of bacterial colonies was observed; and sometimes these bacteria appeared in predominant numbers. The bacteria identifications were performed using standard kits (API 20 Staph and Vitek 2). Sequencing was carried out so as to improve the biochemical identification of isolated bacteria and the evolutionary relationships of their taxa. It was also conducted in order to find the possible source of the fish infection. Comparison of our strains' molecular structures with the data available in GenBank showed that Kocuria rhizophila or Micrococcus luteus had never been isolated from diseased fish before, and that our isolates were very similar to the strains which had been isolated from food processing environments (in the case of Kocuria rhizophila) and from scallops (Micrococcus luteus). The challenge tests performed with our strains of Kocuria and Micrococcus on rainbow trout in laboratory aquaria confirmed the three Koch postulates. Antibacterial disc diffusion studies showed that Kocuria rhizophila and Micrococcus luteus are sensitive to most of the drugs tested i.e. tetracyclines, β-lactams, a macrolide, and amphenicol. The results of these studies show that control of outbreaks of the diseases in rainbow trout or brown trout seems realistic if caused by Kocuria rhizophila or Micrococcus luteus.