The structure of pigeon breast muscle mitochondria was studied both in whole muscle tissue and in mitochondrial fractions separated by differential centrifugation. No evidence was found to indicate that the mitochondria of pigeon breast muscle differ in any fundamental way from those of other tissues. In particular the absence of an enveloping membrane, claimed in recent reports, was not confirmed. The structure of the lipide bodies present in muscle has been described, and compared with that of similar bodies in other tissues.