Acetylcholinesterase (EC 3.1.1.7) activity was demonstrated in whole worm homogenates of adult Ascaridia galli with acetylthiocholine as substrate. The pH optimum was not measurable because of an autohydrolysis of the substrate. The Michaelis constant (Km) was 4 mM with saturation by excess substrate. Optimum enzyme activity was noted at a protein concentration of 200 mg/ml assay medium and at a temperature of 37 degrees C. Arrhenius plot of temperature dependence of the enzyme activity showed an energy of activation (delta Ea) of 28.962 K joule/mole above, and 25.448 K joule/mole below, the transition temperature (37 degrees C). Complete inhibition by eserine (physostigmine), a specific and classical acetylcholinesterase inhibitor, established the identity of the enzyme. A marginally higher enzyme activity was observed in females than in males as well as in homogenates from worms of mixed sexes. The enzyme was markedly activated by divalent metal cations such as Fe2+, Mg2+, Cd2+, Cu2+, Zn2+ and Ca2+, while Co2+ and Mn2+ inhibited the activity. Piperazine adipate at a concentration of 10(-3) M caused 45.5% and albendazole, a benzimidazole anthelmintic, 37.5% inhibition in the enzyme activity, while levamisole and mebendazole proved to be practically ineffective, causing an inhibition of 12 and 9%, respectively.