In rainbow trout sac fry the mixed infection of VHSV and IHNV has been described. The relationship between the presence of these viruses and fry mortality was confirmed, as evidenced by the high virus titer (up to 8,6x10 6 TCID 50 /ml.) in fish tissues and organs, as well as the lack of other potential etiological agents. The presence of rhabdoviruses in sac fry was proved with TEM. Among the typical virus particles, some aberrant-elongated or filamentous and bifurcate forms were also detected. The presence of the viruses was proved using BF-2 and EPC cell lines previously inoculated with the field material, and for its identification the RT-PCR technique was applied for the first time in Poland, both in standard and multiplex modification. VHS virus was also identified with ELISA test and IHN virus with seroneutralisation test. The source of VHS and IHN infection in sac fry has been discussed.