This study was aimed at evaluating the diversity of submerged and emerged flora of Lake Szeląg Wielki and at verifying the hypothesis that a pesticide tomb affects the diversity of submerged and emerged flora of that lake. Investigations carried out in Lake Szeląg Wielki demonstrated a modifying effect of a pesticide tomb on the floral abundance in phytocoenoses of submerged plants: Potamettum perfoliati, Ceratophylletum demersi, Ranunculetum circinati; of emerged plants: Typhetum angustifoliae, Scirpetum lacustris, Phragmitetum, Glycerietum maximae, Equisetetum fluviatile. Phytocoenoses of Scirpetum lacustris detected in Lake Szeląg Wielki were the most susceptible to the activity of a pesticide tomb.