The cattle filarial parasite Setaria digitata is reported to have two ubiquinones, Q(6) and Q(8). These quinones are synthesized within the parasite itself and are not of host origin. Maximum concentration is found in the mitochondria of the parasite. When both Q(6) and Q(8) are formed and present in the adult stage, the microfilarial stage is now shown to contain only one quinone, namely Q(6). Both in the adult and the mf stage, Q(6) is associated with the process of electron transport. Though reduction of oxygen in S. digitata results in the generation of high concentrations of oxidants, antioxidants such as catalase and tocopherol are present in relatively lower concentrations. Hence it is proposed that the higher ubiquinone Q(8) which is not involved in the electron transport process, is functioning as an antioxidant compensating for the reduced levels of classical antioxidants.