Cytochrome components of mitochondria from Taenia crassiceps cysticerci were studied by difference spectrophotometry (at 22 degrees C). The following concentrations of cytochromes (expressed in nmol X mg-1 protein) were found: 0.132 (cytochrome b), 0.083 (cytochrome c), and 0.0856 (cytochrome aa3). The participation of aerobic processes in the general production of energy is discussed and the results are compared with literary data obtained in cytochrome studies of other parasitic worms.