In five cases of dichlofenthion poisoning due to suicidal ingestions, unusual toxicologic features were encountered. Initial symptoms were mild or delayed and severe cholinergic crises did not appear until 40 to 48 hours after ingestion. Two patients died, and in the three survivors, cholinesterase symptoms persisted for five to 48 days. An almost total inhibiton of both cholinesterase enzymes continued in one patient for 66 days, and prolonged antidotal therapy was required. Residues of the intact pesticide were detected in fat after 54 days in one patient, and in the blood for 75 days in another patient. The partition coefficient of dichlofenthion in fat was 20 times greater than parathion, and exceeded only by leptophos. Considering the chemical and toxicologic properties of this compound, we postulated that the mechanism of protracted intoxication is a slow release from adipose reservoirs.