Juvenile Fasciola hepatica were isolated immediately after in vitro emergence from the metacercarial cysts and incubated with uniformly labelled glucose. Under aerobic conditions, carbon dioxide was the main end product of glucose breakdown. In the absence of oxygen, glucose was fermented mainly to propionate and acetate in a molar ratio of 2 : 1, with lactate as a minor product. This anaerobic end-product pattern closely resembles that of the adult liver fluke. In the presence of oxygen and 1 mM cyanide, lactate accumulated. The difference between anaerobic glucose breakdown and that in the presence of cyanide is explained by an inhibitory effect of cyanide on the malic enzyme (EC 1.1.1.40) of the juvenile mitochondria. A substantial Pasteur effect is calculated from these incubations. The oxygen consumption of the juveniles was completely cyanide-sensitive. From these results it is concluded that in aerobic conditions the juvenile liver flukes have an aerobic energy metabolism. Since they can survive prolonged periods of anaerobiosis, they should be called facultative anaerobes.