The objective of this study was to evaluate the efficacy of the synthetic pyrethroid alphamethrin in the control of Rhipicephalus sanguineus in dogs. In vitro tests with filter paper was done to nonengorged larvae, engorged larvae, nonengorged nymphs, engorged nymphs and nonengorged adults of R. sanguineus, given respectively a LC 50 of 3,63;4,79;5,87;10,01 and 11,19 ppm and a LC 90 respectively of 13,17; 17,65; 29,37; 37,32 and 42,99 ppm. Different concentrations of alphamethrin was used in engorged females of R. sanguineus and the concentration of 300 ppm showed the most Reproductive Efficiency percentual (ER) (100%). In vivo tests in six dogs artificially infested with 500 nonengorged larvae, 100 nonengorged nymphs and 30 non engorged adults of R. sanguineus, given 86,2; 100 and 100% of mean efficacy respectively to larvae, nymphs and adults.