In 2003–2006, bacteriophages were for the first time recorded and studied in Lake Baikal with transmission electron microscopy. Bacteriophages were isolated from water samples collected from the lake surface and different depths up to 1400 m. Their morphological composition, as well as size spectrum and their seasonal abundance dynamics were analyzed. A wide range of variability of bacteriophage morphotypes was revealed which were identified according to the international classification: families Siphoviridae – 56 %, Podoviridae – 13 %, and Myoviridae – 7 %. The size spectrum of Baikal bacteriophages is represented by five size classes among which prevail phages of 30–80 nm. Their maximal abundance was recorded in the 0–25 m water layer, while seasonal dynamics had two peaks of the abundance – spring and summer-autumn. The periodicity of bacteriophage development is in good correlation with seasonal dynamics of the total abundance of bacterioplankton of the lake.