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The effect of the herbicide glyphosate (N-(phosphonomethyl)glycine) on the growth, respiration and nitrogen fixation of Azotobacter chroococcum and A. vinelandii was studied. Azotobacter vinelandii was more sensitive to glyphosate toxicity than A. chroococcum. Recommended dosages of glyphosate did not affect growth rates. More than 4 kg ha-1 is needed to find some inhibitory effect. Specific respiration rates were 19.17 mmol O2 h-1 g-1 dry weight for A. chroococcum and 12.09 mmol h-1 g-1 for A. vinelandii. When 20 kg ha-1 was used with A. vinelandii, respiration rates were inhibited 60%, the similar percentage inhibition A. chroococcum showed at 28 kg ha-1. Nitrogen fixation dropped drastically 80% with 20 kg ha-1 in A. vinelandii and 98% with 28 kg ha-1 in A. chroococcum. Cell size as determined by electron microscopy decreased in the presence of glyphosate, probably because glyphosate induces amino acid depletion and reduces or stops protein synthesis.