Summary.Laboratory experiments were conducted to determine if paraquat had any effect on soil micro‐organisms and consequently on soil fertility.Numbers of bacteria and moulds tended to increase with incubation time as residual paraquat decreased. Lag periods of 1–5 days were observed in the development of soil microflora and concomitant degradation of paraquat. Under controlled culture conditions,Aerobacter aerogenes, Agrobacterium tumefaciens, Pseudomonas fluorescensandBacillus cereuswere able to utilize paraquat as sole carbon and nitrogen sources in synthetic media.Paraquat at 0–25, 0–5, 2–5 and 25 ppm cation was added to four different soils to study changes in microbial activities concomitant with decreases in herbicide concentration. Analytical recoveries were correlated inversely with clay mineral content, cation‐exchange capacity and organic matter.Mineralization of peptone nitrogen by amnnonification and nitrification was stimulated by 0–25 ppm; higher rates up to 25 ppm had little influence. A bimodal effect or growth inversion was observed in several instances, with increased microbial development at intermediate concentrations. Paraquat at recommended field rates appeared to have no appreciable influence on general microbial activities of importance to soil fertility.Interaction entre U paraquat et les microbes