We used specific diversity of phytoplankton and chlorophyll concentrations as a measure of algal biomass in ponds located in agricultural areas where pesticides and fertilizers are currently used and in natural meadows where pesticides spraying has never occurred .A comparison of the biocoenotic structure of the primary producers in the two types of ponds shows conspicuous differences . Species richness is higher in meadow ponds (25 species) than in agricultural area ponds (10 species) . The chlorophyll levels are reduced in agricultural area ponds . These effects could be ascribed to contamination by pesticides . 269