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Modifications of Azospirillum lipoferum Sp Br17 were observed when it was incubated with the organochloride insecticide Dicofol. In experiments run under non N2 fixing conditions, cellular volume measured in a Coulter Counter Channalyser was reduced immediately after addition of pesticide. Cell counts, made in a Coulter Counter increased during 12 hours after addition of 250 and 500 ppm of Dicofol. Cell viability was reduced by 1 hour incubation in Dicofol concentrations of 10 to 500 ppm, These results suggested that one of the first consequences of Dicofol action was cell disruption, Incubation for 1 to 7 days showed that maximum cell counts, protein content, average cell volume, and viability were lowered by increased Dicofol concentrations. Growth was totally inhibited by 250 and 500 ppm of Dicofol but in some experiments a latent growth was observed, suggesting selection of resistant mutants. Experiments under N2 fixing conditions showed that nitrogenase activity was inhibited by increasing Dicofol concentrations of up to 1000 ppm, but in some experiments was stimulated by a concentration of 1 ppm, Microscopic obser vations of Azospirillum morphology in the presence of Dicofol showed cyst like formations that may be an induced resistance mechanism.