Rhipicephalus sanguineus, the brown dog tick or kennel tick, is a three-host ixodid tick that is widely distributed throughout tropical and temperate climates. R. sanguineus is somewhat unusual among the ixodids in that it can tolerate low humidity, survive for long periods of time indoors; in the absence of routine acaricide use on dogs, severe home infestations with R. sanguineus may develop, and persist as long as canine hosts are available to support development and reproduction of the ticks. [1] Although dogs are the most common host of this tick [2], it has also been recovered from other animals, such as cats, rabbits, camels, bovines in general, goats, horses, sheep, bats, reptiles, and ground feeding birds and humans [3]. For health problem, R.sanguineus is a species of both veterinary and medical importance. It causes blood loss in the host and transmits several pathogens such as Babesia canis and Ehrlichia canis to dogs and Rickettsia conori to humans, but regional differences seem to exist in vectoring capacity [3]. In the present study the external morphology of brown dog tick (R.sanguineus) has been examined by scanning and transmission electron microscopy. In scanning micrographs fine morphological organs have been observed in two life stages (nymph and adult). Many fine morphological organs were detected among theR.sanguineus collected at the two localities, such as body size, shape of the genital pore, and morphology of the sensory structures. All these characteristics are fundamental for the diagnosis of species of the genus Rhipicephalus and thus indicate the need for further the taxonomical revision of this species of tick in the Thailand.