The use of mirex [1,3,4‐methenododecachlorooctahydro‐2H‐cyclobuta (c,d) pentalene] for the control of the imported fire ant in pasture land is a potential source of contamination of meat and milk of dairy animals. To determine the level and nature of the residues which might occur should the animals consume the insecticide in the diet, mirex‐14C was administered via gelatin capsules twice daily to a lactating Jersey cow for 28 days. The daily dose was equivalent to approximately 0.2 ppm mirex in the diet. Radioactive residues in the feces reached a maximum of 50% of the consumed dose after 3 weeks and remained at that level until treatment was terminated. An additional 3% was eliminated in the feces during the 28‐day post‐treatment period. Mirex‐14C equivalents in the milk reached an equilibrium after about 1 week of treatment. Approximately 10% of the administered doses were eliminated daily by this route with the concentration in the whole milk after equilibration being 0.058 ppm. One week after removing the mirex source, the residues in the milk dropped to 0.006 ppm and then declined gradually to 0.002 ppm after 28 days. Residue levels in the tissues 28 days after the final treatment were: fat, 0.21 ppm; liver, 0.03 ppm; and kidney, 0.02 ppm; the muscle and brain contained no detectable residues. TLC (thin‐layer chromatography) and GLC (gas‐liquid chromatography) analyses of the milk, feces, and fat demonstrated that unchanged mirex‐14C was the only radioactive component of these substrates.