In field and pot experiments the effect of selected pesticides on the atmospheric nitrogen fixation, number of microorganisms in soil and yield of red clover has been investigated. The results obtained indicate that crop protection preparations applied in the experiments (Funaben T seed dressing compound and Pivot 100 SL herbicide) resulted in reduction of nitrogenase activity on the active strain of Rhizobium leguminosarum bv. trifolii KGL both in pot and field experiment conditions. Moreover, the authors observed the toxic effect of the pesticides used on nodulation, root development and yield of clover. The herbicide and fungicide applied also inhibited the multiplication of the microorganisms in soil under red clover plantations in the first days upon application and, later on, stimulated their multiplication.