IN nematode parasites the anaerobic breakdown of glycogen to pyruvate and lactate with a consequent formation of adenosine triphosphate follows a course similar to that in vertebrate muscle1. Since it appears that aerobic mechanisms may play an important part in the metabolism of certain nematode parasites2, studies on the carbohydrate metabolism of the parasites have been extended to include some aspects of the oxidative utilization of pyruvate. The organisms used were Ascaridia galli from the chicken small intestine and Nematodirus spathiger and N. filicollis from the sheep small intestine. Some experiments were also carried out with Neoaplectana glaseri, a parasite of the Japanese beetle, which, however, was grown in vitro for the present experiments. Owing to its relatively large size and its position in the host, A. galli may not have an active aerobic metabolism in vivo. However, it has been shown that Nematodirus spp. probably respire actively in vivo, and oxygen is essential for the survival in vitro of Neoaplectana glaseri.