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