Populations of Simulium (Chirostilbia) pertinax Kollar, 1832 from Southern and Southeastern States of Brazil were analyzed for temephos susceptibility considering control historical information and possible resistance. In situ bioassays were carried out for populations from the states of Paraná (Tibaji and Rolândia), Rio de Janeiro (Muriqui) and São Paulo (Barra do Una, Ilhabela and Morungaba). The populations were characterized as susceptible (S) or resistant (R) by submitting larvae in the last instars to an operational concentration (0.1ppm a.i./10min) of temephos (Abate 500E) as diagnostic. The possible mechanisms for the organophosphorus resistance development are discussed considering old and new control strategies.18661-71