“Tick paralysis” occurs in British Columbia and affects man, sheep, and probably other animals. The disease is caused by the bites of Dermacentor venustus Banks. It is usually of short duration, is benign in character, but occasionally it persists for long periods, and may terminate fatally. From an economic point of view, the disease is of some importance to the sheep industry. The causative agent has not been discovered, and the disease has not been reproduced by inoculation. The most likely hypothesis is that the tick injects a toxin which gives rise to symptoms appearing coincidentally with the complete engorgement of the tick. In three consecutive cases, experimentally produced by me in lambs, paralysis occurred six to seven days after the ticks were put on. In no case did I fail to produce paralysis through the agency of the tick bites. It has been proved that D. venustus usually bites sheep along the back-bone; possibly the point of attachment may have some bearing on the symptoms or severity of the case. It is probable that other species of ticks may produce a similar disease. No larvae or nymphs were encountered on sheep, and I have no records of their attacking man in British Columbia.