Recent evidence from the biochemical, the genetic and the morphologic study of bacteria, in that chronological order, has indicated essential similarities of the bacterial cell to the cells of higher organisms. Recognition in bacteria of a large category of cytoplasmic granules as possessing characteristics which strongly suggest that they are the functional equivalents of the mitochondria of anirnaE and plant cells, has been a significant step. It is the purpose of this communication to summarize the properties of these cytoplasmic granules in bacteria, in so far as they have been worked out, and thus to arrive at a definition of the mitochondria of bacteria. Morphology. In electron micrographs under conditions in which the surround-