Enzymes effecting aldrin epoxidation and dihydroisodrin hydroxylation were studied in various tissues of Gromphadrohina portentosa Schaum, and optimal in vitro assay conditions were established. Enzyme activity was associated with the midgut, gastric caeca, fat body, and malpighian tubules and only slightly with the hindgut and crop. The enzymes in the midgut and caeca required NADPH and oxygen for optimum activity and were inhibited completely by carbon monoxide. Electron micrographs of various subcellular fractions from the midgut and caeca showed that enzyme activity was associated with a fraction comprising membranous vesicles derived from both the rough and smooth endoplasmic reticulum. Large variations in enzyme activity were observed through several in stars, and activity was lowest during the internymphal molts. These changes in enzyme activity correlate favorably with the toxicity of propoxur to cockroaches of different ages.