The fine structure of the mitochondria ofAmoeba proteus was studied utilizing several methods of fixation. Different forms of mitochondria were detected within the same ameba following aldehyde fixation. Morphological differences between them were most evident in cells fixed withKarnovsky's glutaraldehyde-formaldehyde mixture. Mitochondria with a dense matrix appeared to be rod-shaped and had tubular cristae of variable width. In comparison, other mitochondria had a lighter matrix, an irregular spherical shape, and narrower more uniform tubular cristae. A small percentage of mitochondria with intermediate characteristics was noted. The results obtained with the various preparative procedures are compared and the possible significance of the morphological types is discussed.