Parasites have developed a variety of physiological functions necessary for their survival within the specialized environment of the host. Using metabolic systems that are very different from those of the host, they can adapt to low oxygen tension present within the host animals. Most parasites do not use the oxygen available within the host to generate ATP, but rather employ anaerobic metabolic pathways. In addition, all parasites have a life cycle. In many cases, the parasite employs aerobic metabolism during its free-living stage outside the host. In such systems, parasite mitochondria play diverse roles. In particular, marked changes in the morphology and components of the mitochondria during the life cycle are very interesting elements of biological processes such as developmental control and environmental adaptation. Recent research on the respiratory chain of the parasitic helminth Ascaris suum has shown that the mitochondrial NADH-fumarate reductase system plays an important role in the anaerobic energy metabolism of adult parasites inhabiting hosts, as well as describing unique features of the developmental changes that occur during its life cycle.