Conditions favouring the sharp increase of risk to be infected with cercariasis (caused by trematode cercariae Trichobilharzia ocellata Brumpt, 1931, Schistosomatidae) were examined in Moscow region (Russian Federation). Main factors of worsening the cercariasis situation are as follows. a) The increase of environment pollution by everyday wastages and the overgrowing with macrophytes in internal water basins of Moscow, that makes favourable conditions for the development of molluscs, which are intermediate hosts (mainly Lymnaea ovata and L. auricularia). b) The sharp increase of number of ducks (mainly mallard Anas platyrhyncos), which are final hosts, that is a result of bird escaping from farms (approximately in 60-70-th) and of high adaptation of these birds to city water basins. The detailed estimation of the cercariasis situation in 89 water basins of different types in Moscow and recreation zone was carried out. Examples of charting the zones with different risk degree of cercariasis infection are given. It is stated that at current time the cercariasis became a significant and widely distributed medical problem, which is most important in cities (including such large one as Moscow). Prophylaxis recommendations are proposed.