The distribution of fumarase and malic enzyme in Ascaris suum muscle mitochondria was investigated by employing digitonin fractionation techniques. The ability of these procedures to resolve the various submitochondrial fractions (intracristal space, inner membrane, matrix and inner membrane-matrix particles) was verified by electron microscopy and the distribution of appropriate marker enzymes. From the data obtained, it is concluded that fumarase in Ascaris muscle mitochondria is located solely within the matrix compartment and is not present within the intracristal space as reported earlier by other authors (Rew and Saz (1974) J. Cell. Biol. 63, 125-135). In agreement with previous findings malic enzyme in the nematode organelle appears to be associated with both soluble compartments. The implications of these findings to the parasites' mitochondrial metabolism are discussed.