Topically applied heptachlor is rapidly absorbed by heptachlor-resistant house flies and is metabolized to heptachlor epoxide. Identification of the metabolite was made by colorimetric analysis using potassium hydroxide-diethanolamine reagent, and by infrared spectroscopy. The metabolite was found to be as toxic as heptachlor when bioassayed against susceptible house flies. Male flies of the resistant strain are considerably more susceptible to heptachlor than females, yet very little heptachlor was found in the tissues of flies which succumbed to a dosage of 8 [mu]g of heptachlor per fly. Susceptible flies also convert heptachlor to the epoxide and the onset of symptoms of poisoning coincides with the appearance of the metabolite. This suggests that death might result from the toxic action of the metabolite and not from the direct effect of heptachlor. Recovery of products resulting from heptachlor metabolism was in no case quantitative and the missing portion has not been characterized. The mechanism of resistance to heptachlor appears to be that of inactivation of the toxic metabolite by solubilization or storage in a non-sensitive tissue such as fat body.