In field experiment the effect of two active ingredients (imazethapyr and linuron) on symbiotic nitrogen fixation activity and microorganisms under legume crops (pea, horse bean, yellow lupine, white lupine, soybean) was determined. The studies indicated that both imazethapyr and linuron can cause decrease of root - nodule bacteria nitrogenase activity. They also can stimulate development of bacteria and inhibit growth of fungi.