The current list of planktonic ciliates dwelling in Lake Baikal numbers 102 species, 81 of which are considered as euplanktonic. This plankton ciliate community is characterised by high percentages of oli-gotrichs and gymnostomes, 28 and 27%, respectively. One third of these ciliates are cold-stenothermic species, with temperature optimum preference of 0-4 °C, including all the species believed endemic. They develop only in under-ice plankton though remaining present in the water column until June-July. The other two thirds of planktonic ciliates are eurythermic and widespread in lakes of the Northern Hemisphere or cosmopolitan. Two maxima occur annually in baikalian planktonic ciliates: the first in March-April and the other in August-September. They coincide with the annual maximal development of phytoplankton. During years of low to middle-level primary production, the summer maximum of protozooplankton has been higher than the spring one. But the latter can exceed the former in years of high primary production. Mean biomass of ciliates (in 0-50 m) reaches 140-500 mg m -3 during maximum abundance, decreasing by 5-10 times in the rest of the year. Mixotrophic ciliates contribute up to 6% of the under-ice pelagic ciliate community but sharply increase to 80-90% in summer, augmenting the autotrophic plankton biomass by 30-40%. In the last decade, pelagic ciliates have seldom exceeded 4-5 thousand cells l -1 at maximum development, averaging 1-2 thousand cells l -1 . The dominant role of nanooligitrichs and high percentage of mixotrophic ciliates are characteristic of summer plankton of Baikal. Planktonic ciliates in interstitial water of melting ice also make a significant contribution to total abundance. Their rapid increase in population, which can exceed 10 thousand cells l -1 , is another peculiarity. It is thought that inter-annual dynamics of the ciliates are related with long-term cycles of baikalian phytoplankton.