A study of thin sections of Paramecium multimicronucleatum, Tetrahymena pyriformis, Tokophrya infusionum, and Amoeba proteus shows that the mitochondria in all these protozoa are similar in certain aspects of their fine structure to that described in metazoan cells. As in higher organisms the mitochondrion is surrounded by a double limiting membrane and contains protrusions directed inward from the innermost of the double membranes. There are, however, some differences. In a majority of higher organisms the internal structure of mitochondria consists of ridges or cristae mitochondriales and in a few instances only of finger-like projections, or microvilli. In all protozoa described here and elsewhere microvilli represent the dominant structure. They are characteristic therefore of protozoan mitochondria.