In the spring and autumn of 1999-2000, the physical and chemical properties of water and the qualitative and quantitative structure of phytoplankton as well as its productivity based on the concentration of chlorophyll a in the littoral and pelagial of Lake Płotycze near Urszulin and Lake Wereszczyńskie were studied. The phytoplankton density in Lake Płotycze ranged from 212.6 thousand to 681.5 thousand per dm3. In the spring the most numerous was Peridinium willei, while in the autumn Cryptomonas sp. The species diversity of the phytoplankton was higher in the spring. In both years, throughout a season mean concentration of chlorophyll a was much higher in the summer (8.61–23.79 mg m3) than in the autumn (3.57–5.04 mg m3), especially in 2000. Mean annual chlorophyll a count both in the littoral and pelagial were much higher in 2000 (16.09 and 12.74 mg m3 respectively) than in 1999 (6.05 and 6.14 mg m3 respectively). In Lake Wereszczyńskie, the phytoplankton density ranged from 266.5 thousand to 894.2 thousand per dm3 and was higher than in the littoral. In all occasions, Cryptomonas sp was found to dominate. The species diversity was higher in the autumn. In 1999, concentration of chlorophyll a was not much different between seasons, while in 2000 it was much higher during the autumn. Except of the spring of 1999, concentration of chlorophyll was higher in the littoral. The annual mean concentration of chlorophyll a was 7.01 mg m-3 and 4.43 mg m-3 in 1999 and 2000, respectively. Based on chlorophyll a concentration, Lake Płotycze was ranked to trophic status between mesotrophy and eutrophy whereas Lake Wereszczyńskie was a mesotrophic lake.