Abstract: This paper presented the cholinesterase (ChE) activity value of Necator americanus (Na), Ancylostoma duodenale (Ad), Ancylostoma caninum (Ac) and Nippostrongylus braziliensis (Nb). The ChE activity of Na was about 40 times higher than that of Ad, The ChE activity of Ad was close to that of Ac while the ChE activity of Nb was approximately one half of that found in Na. Various anti-hookworm drugs including metrifonate, bephenium hydroxynaphthoate, mebendazole, pyrantel pamoate, tetrachloroe-thylene and amidental were given orally to hamsters, puppies and rats infected either with Na, Ad, Ac or Nb to study their effects on the ChE activity of these worms.Metrifonate administered at a single dose of 150mg/kg inhibited completely the ChE activity of adult Na in hamsters as well as that of adult Ad and Ac in puppies. The inhibition of ChE activity by metrifonate in Na was reversible.Bephenium hydroxynaphthoate had no effect on ChE activity of Na in hamsters at a dosage of 3g/kg. However, it inhibited partially the ChE activity of Ad in puppies 24 hours after administration.Pyrantel pamoate, mebendazole, tetrachloroethylene and amidentel had no effect on ChE activity of Na and Ac.