Studies performed on sheep showed that Rhipicephalus evertsi mimeticus is a paralysis-competent tick species whose toxicity is either the same as, or only slightly less than that of R.e. evertsi. It was also proved that the paresis or paralysis induced by the mimeticus females is characterized by a constant incubation period which is independent of the tick infestation rate and body mass of the host, and consequently of its age. This period is entirely determined by the state of repletion of the ticks and is regulated by the date of mating. It was further proved that a precise relationship exists between an increase of engorgement or salivation activity and the intensity as well as the persistence of clinical symptoms. Thus only female ticks in the mass range of 15-21 mg are toxic. Strain-dependent differences could not be demonstrated. The infestation rates of subspecific hybrid female ticks, and of their descendants, that are necessary to produce the respective symptoms showed no differences in toxicity. When infestations were limited to a small skin area minimum paresis could frequently not be induced, even though the actual number of ticks applied was sufficient to cause lethal paralysis.