Body lice (Pediculus humanus corporis L) and ticks (Ornithodoros moubata) that had been infected with Rickettsia prowazekii received varying doses of erythromycin. The anti-rickettsial activity of this drug was evaluated with regard to the determination of the numbers of surviving microorganism (LID100) and the in vivo concentration of erythromycin in both arthropod hosts. Erythromycin was found to exert a rickettsiostatic effect in lice, whereas in ticks the drug at concentrations of 0.3–0.04 μg/mg body weight completely eliminated R. prowazekii