We wished to evaluate the safety and anthelmintic efficacy for man of dichlorvos (Dichlorman®), a new anthelmintic compound. Clinical, parasitologic, and pharmacologic studies were conducted on 108 hospitalized adult patients. The drug was administered in a granular-resin formulation that permits slow liberation in the intestinal tract. Formalinether (MGL) and Stoll egg-count techniques were employed for evaluation of therapy at different dosage levels. A wide range of laboratory studies was done, with particular emphasis on plasma and red-cell cholinesterase determinations. Despite the fact that the drug caused reduction in plasma cholinesterase levels, no clinical side-effects were observed in the carefully monitored patients, except for brief mild headache in a few. Cure rates of infections, with a single oral dose of 12 mg per kilogram of body weight, were 85.7% for hookworm, 87.8% for Trichuris trichiura, and 77.8% for Ascaris lumbricoides; egg-count reductions in the cases not completely cured were 97.3, 98.5, and 83.7 percent, respectively.