Phosphorylation in isolated Ascaris suum mitochondria was much greater in the presence of malate than succinate, but, in the absence of added adenine nucleotides, incubations in succinate resulted in substantial elevations in intramitochondrial ATP levels. Succinate-dependent phosphorylation was stimulated aerobically and this stimulation was due almost entirely to a site I, rotenone-sensitive, phosphorylation. Increased substrate level phosphorylation, coupled to propionate formation, or additional sites of electron-transport associated ATP synthesis were not significant. Under aerobic conditions, 14CO2 evolution from 1,4-[14C]succinate was stimulated and NADH/NAD+ ratios were elevated, but the formation of [14C]propionate was unchanged. It appears that succinate was metabolized to pyruvate and acetate, and NADH, generated from the decarboxylations of malate and pyruvate, was the primary source of reducing power fueling electron-transport. The terminal oxidase and final electron-acceptor are still not clearly defined. However, ferricyanide, H2O2, and 100% oxygen all stimulated succinate-dependent phosphorylation. A possible role for cytochrome c peroxidase in A. suum mitochondrial metabolism is discussed.