The mitochondrial electron transport system of adult Hymenolepis diminuta exhibited an apparent specificity in terms of reduced pyridine nucleotide utilization. The preferred substrate for both the minor oxidase and the physiologically required fumarate reductase system was NADH. Intramitochondrial reducing equivalents, needed for phosphorylation via the anaerobic, electron transport-dependent, fumarate reductase, were generated as NADPH by the action of the cestode's NADP-specific "malic" enzyme. However, H. diminuta mitochondria catalyzed an NADPH : NAD transhydrogenation which would serve in hydride ion transfer from NADPH to NAD, thereby producing NADH required for the anaerobic, electron transport mechanism. Accordingly, NADPH utilization was increased when NAD was added to the mitochondrial system. The most significant increase occurred in the presence of both NAD and fumarate. These data indicate a coupling of the NADPH : NAD transhydrogenase with mitochondrial electron transport. This coupling of the transhydrogenase with electron transport was demonstrated using disrupted mitochondria and mitochondrial membrane preparations. Under conditions of reduced oxygen tension, the coupling of the transhydrogenase to fumarate reduction was apparent. In adult Ascaris suum, where the "malic" enzyme physiologically utilizes NAD, the mitochondria differ from those of H. diminuta because NADPH : NAD transhydrogenase activity was minimal under the conditions of assay. The rate of NADPH utilization by the nematode mitochondrial system is not increased appreciably in the presence of NAD when either oxygen or fumarate serves as the acceptor.