Cholinesterase activity of organophosphorus-susceptible and resistant cattle ticks (Boophilus microplus) has been determined, and found to be much lower in the resistant strain. Experiments on inhibition of tick cholinesterases by organophosphates indicated that the resistant strain possessed at least one cholinesterase which reacted more slowly with organophosphates than did the cholinesterase of the susceptible strain. The greatest difference of reaction rate occurred with compounds which give the largest factors of resistance under bio-assay conditions. The organophosphate resistant cholinesterase was also less rapidly inhibited by carbamates. A strain of the blue tick (B. decoloratus), which is resistant to arsenic and chlorinated hydrocarbon insecticides, had normal levels of cholinesterase activity, and this enzyme was fully susceptible to organophosphate inhibition.