A b s t r a c tA study was carried out to determine the effects of paraquat, pretilachlor and 2, 4 D on growth and nitrogen fixing activity of Stenotrophomonas maltophilia (Sb16) and pH of Jensen's N free medium. The growth of Sb16 and pH of medium were significantly reduced with full (X) and double (2X) doses of tested herbicides, but nitrogen fixing activity was decreased by 2X doses. The nitrogenase activity had the highest value in samples treated with 1/2X of 2, 4 D on fifth incubation day, but 2X of 2, 4 D had the most adverse effect. An inhibition in the growth and nitrogenase activity was recovered on the last days of incubation. 378activity of Sb16 and pH of Jensen's N free medium (Jensen, 1951) within a determined incubation time.Medium was inoculated with a colony of Sb16 strain and incubated at 35°C on a rotary shaker at 150 rpm for 3 days. Aliquots of 1 ml of approximately 10 8 cfu/ml of live cells, with adjusted optical density (OD) 600 of ≈ 0.1, was inoculated to each culture flask.The herbicides used in this study were paraquat dichloride (13% w/w) Syngenta Capayam, pretila chlor (28.7% w/w) Syngenta Sofit N300 EC and 2, 4 D isopropylamine (28% equivalent) (35.5 % w/w) Kompressor Ancom Cropcare. The herbicides solu tions were prepared by mixing the required amount of active ingredient in sterilised distilled water to obtain concentrations corresponding to 0, 1/2, 1 and 2 times of the recommended field application rate. Therefore, four rates of 0, 0.78, 1.56 and 3.12 mg/ml of paraquat at the rate of 12 g/l; 0, 0.72, 1.44 and 2.87 mg/ml of pre tilachlor at 5 g/l; 0, 1.42, 2.84 and 5.68 mg/ml of 2, 4 D at 8 g/l were prepared to get the concentrations of 0, 1/2, 1 and 2 times of the recommended field application rate, respectively. Thereafter, herbicides were sterilised by filtration (Millipore filter, 0.22 µm) aseptically in a laminar flow cabinet.The prepared herbicides solutions were added to each sterilised flask (250 ml) containing 75 ml Jensen's N free broth medium. Control flasks were without herbicides. Before inoculum application, optical density (OD 600 ) of inoculum was checked and regulated to approximately 0.1 and the drop plate method for cell count on N free agar was employed to confirm the popu lation (Somase garan and Hoben, 1985). Aliquots of 1 ml of the desired inoculum (approximately 10 8 cfu/ml) of live cells was transferred to each culture flask using sterilised pipette. The flasks were incubated at 28°C on a rotary shaker for 7 days till the end of the experiment.At each sampling period, 1 ml of the culture was sampled and 10 fold serial dilutions were made up to 10 -8 . The mixture in each test tube was shaken vigor ously to suspend bacterial cells. Aliquots of 0.1 ml of appropriate dilutions was placed on each Jensen's agar plate. The plates were then incubated at 32°C. The popu lation was determined using the drop plate method at 24 h intervals within 7 days.The N 2 fixing activity of Sb16 strain in Jensen's N free broth, amended with paraquat, pretilachlor and 2, 4 D, wa...

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