SUMMARYMitochondrial configurations of various members of the Mycetozoa were examined. It was found that all members of the Eumycetozoa (amoebae with filose pseudopodia) had tubular mitochondrial cristae. Members of the Acrasea (amoebae with lobose pseudopodia) which show differentiation into stalk and sorus were found to have plate-like mitochondrial cristae. Those acrasids with the columnar type of fruiting body and with no stalk-sorus differentiation were found to have tubular mitochondrial cristae. The mitochondrial configurations found in Oomycetes, Hyphochytridiomycetes, and other fungi are briefly noted in reference to their phylogenetic significance in conjunction with biochemical parameters. Unusual helical structures found in the mitochondria of certain isolates of the Acrasea are discussed.