Frillneck lizards (Chlamydosaurus kingii) in northern Australia are frequently infected by a mosquito-borne filarial parasite, Oswaldofilaria chlamydosauri. Two sites were studied and the parasite was found to be common at one but absent at the other. At the site where the parasite was present, larger lizards had a higher prevalence of infection. The number of microfilariae in the blood of the lizards was not related to blood hematocrit or hemoglobin concentration. An index of body condition was not related to the presence or number of microfilariae in the blood. Maximum rates of oxygen consumption were measured by running the lizards on a treadmill and neither the presence nor the number of microfilariae was significantly related to aerobic capacity. We also analyzed the blood and physiological parameters with respect to snout–vent length (as an indication of age) to test for consequences of chronic infection, but we found no significant relationships. Although only a few physiological parameters have been examined, the available evidence does not indicate any detrimental effects of the filarial worms on the lizards in this host–parasite association.