Phytoplankton structure was studied in five ponds (located in the Oleksandriya Nature Park, Ukraine), which significantly differed in the level of their contamination. The concentration of ammonium (NH4+-N) in the studied water bodies was 0.02–74.00 mg l-1, of nitrite (NO2--N) – 0.002–1.750 mg l-1, nitrate (NO3--N) – 0.13–58.00 mg l-1, inorganic compounds of phosphorus (Pinorg) – 0.041–0.160 mg l-1, chloride (Cl–) – 35.4–560.5 mg l-1, whereas the content of organic matter (PO and DO) – 4.4–10.4 and 18.0–81.0 mg O l-1, respectively. It has been found that the response of phytoplankton to heavy pollution consisted in changes in its species richness, taxonomic structure, species composition, quantitative indices, numbers and biomass structure, dimensional structure, pigment index, dominant complex, as well as changes in its ecological spectrum. Individual divisions of algae differed in their response to heavy pollution of water bodies, which was conditioned by the specificity of algae metabolism. Bacillariophyta and Cyanoprokaryota proved to be more sensitive to contamination, whereas Chlorophyta and Euglenophyta – more tolerant. The obtained data can be used to monitor the status of water bodies and their biota and to determine the type and intensity of contamination.