The article gives the results of integrated studies of bottom sediments in four watercourses in the central St. Petersburg: the rivers of Ekateringofka, Okhta, Fontanka, and Chernaya Rechka. The river sediments are shown to be highly polluted by heavy metals and polycyclic aromatic hydrocarbons. Bioassay tests are used to show a considerable toxicity of sediments with elements of HM group (Ni, Zn, Cu, Cr, Mn, Cd) with a high probability of biological effects. However, studies by correlation and regression analysis have not demonstrated the contribution of polyarenes to the formation of sediment toxicity. This result is in agreement with estimates of sediment toxicity by PAH concentration converted to total organic carbon, and by characteristics of potential risk of biological effects TEC (Threshold Effects Concentrations), MEC (Median Effects Concentrations) and EEC (Extreme Effects Concentrations). Among the examined watercourses, of greatest hazard are the sediments of the Chernaya Rechka and Ol'khovka, especially, near wastewater discharge sites of plants of chemical (paint and varnish) industry and accumulator battery production.