1. Insoluble and "soluble" forms of cytochrome c oxidase were prepared from house flies by methods described for mammalian tissue. A 4.5-fold purification was accomplished by preparing the enzyme in insoluble form (as compared with a crude water brei). The oxidase in "soluble" form was purified 20.0-fold. 2. The absorption spectrum of the "soluble" preparation from flies was similar to that from mammalian tissue, and indicated the presence of cytochromes a + a(3) and b. In contrast with the corresponding mammalian enzymes these components were not separable by the existing techniques. 3. The Soret peak seen at 440 mmicro and the alpha peak at 600 mmicro were due. primarily to cytochromes a(3) and a, respectively. 4. The oxidase activity of whole house flies was comparable with that of rat abdominal muscle. In both organisms, this activity was inhibited by cyanide to approximately the same extent. 5. The optimum buffer concentration for oxidase activity, as measured spectrophotometrically, was approximately 0.02 to 0.06 M. This is in agreement with results obtained for mammalian enzyme, as measured both manometrically and spectrophotometrically. 6. The present study has demonstrated the following components of the cytochrome system in house flies: b, c, a, and a(3).