Mitochondria from the parasitic helminth, Hymenolepis diminuta, catalyzed both NADPH:NAD + and NADH:NADP + transhydrogenase reactions which were demonstrable employing the appropriate acetylpyridine nucleotide derivative as the hydride ion acceptor. Thionicotinamide NAD + would not serve as the oxidant in the former reaction. Under the assay conditions employed, neither reaction was energy linked, and the NADPH:NAD + system was approximately five times more active than the NADH:NADP + system. The NADH:NADP + reaction was inhibited by phosphate and imidazole buffers, EDTA, and adenyl nucleotides, while the NADPH:NAD + reaction was inhibited only slightly by imidazole and unaffected by EDTA and adenyl nucleotides. Enzyme coupling techniques revealed that both transhydrogenase systems functioned when the appropriate physiological pyridine nucleotide was the hydride ion acceptor. An NADH:NAD + transhydrogenase system, which was unaffected by EDTA, or adenyl nucleotides, also was demonstrable in the mitochondria of H. diminuta. Saturation kinetics indicated that the NADH:NAD + reaction was the product of an independent enzyme system. Mitochondria derived from another parasitic helminth, Ascaris lumbricoides, catalyzed only a single transhydrogenase reaction, i.e., the NADH:NAD + activity. Transhydrogenase systems from both parasites were essentially membrane bound and localized on the inner mitochondrial membrane. Physiologically, the NADPH:NAD + transhydrogenase of H. diminuta may serve to couple the intramitochondrial metabolism of malate (via an NADP linked “malic” enzyme) to the anaerobic NADH-dependent ATP-generating fumarate reductase system. In A. lumbricoides, where the intramitochondrial metabolism of malate depends on an NAD-linked “malic” enzyme which is localized primarily in the intermembrane space, the NADH:NAD + transhydrogenase activity may serve physiologically in the translocation of hydride ions across the inner membrane to the anaerobic energy-generating fumarate reductase system.