The impact of diflubenzuron (DFB) on Tetrahymena pyriformis, a ciliated protiste was studied. The treatment with DFB at concentrations of 10 and 20 μg/mL could appreciably reduce the growth of the protiste. The surveillance of protozoa′s respiratory metabolism with polarography technique showed a sensitive inhibition of its oxygen consumption, and at 20 μg/mL concentration the oxygen consumption was increased. For any malformation of morphology and perturbation in swimming trajectory, the neutral red coloration was done. This technique (neutral red) showed that there was a toxic effect of DFB on T. pyriformis, especially at the highest concentration; this toxicity was translated by rotatory swimming in same place and the increasing of coloration quantity in digestive vacuoles. Fig 5, Ref 15 The impact of diflubenzuron (DFB) on Tetrahymena pyriformis, a ciliated protiste was studied. The treatment with DFB at concentrations of 10 and 20 μg / mL could appreciably reduce the growth of the protiste. The surveillance of protozoa’s respiratory metabolism with polarography technique showed a sensitive inhibition of its oxygen consumption, and at 20 μg / mL concentration the oxygen consumption was increased. For any malformation of morphology and perturbation in swimming trajectory, the neutral red coloration was done. This technique (neutral red) showed that there was a toxic effect of DFB on T. pyriformis, especially at the highest concentration; this toxicity could translated by rotatory swimming in same place and the increasing of coloration quantity in digestive vacuoles. Fig 5, Ref 15