Cetrimide (R) has been efficiently assessed against hydatid protoscolices as a scolicidal adjunct to hydatid surgery. It was tested in four concentrations (0.05, 0.1, 0.5 and 1%) against the protoscolices of hydatid cysts in sheep and cattle and in 0.1% concentration against the protoscolices of hydatid cysts in humans. For each concentration, three incubations of one, five and 10 min exposure were tested in vitro and in vivo on sheep and cattle cysts, and two successive incubations each of five min exposure on human cysts. All experiments resulted in rapid mortality of the protoscolices, as judged by their loss of motility, readiness to take vital stains and inability to develop into secondary hydatid cysts when inoculated into albino mice. The low toxicity of Cetrimide (R), its rapid action against the protoscolices, its scolicidal effect at a low concentration, and its low degree of absorption by the host tissues make it the most promising drug in hydatid surgery, displaying no side effects on the host.