Parasites can affect the locomotor performance of their hosts via a range of mechanisms. Cane toads (Rhinella marina) in Australia often contain native-range lung nematodes (Rhabdias pseudosphaerocephala), and the parasite reduces endurance (and thus, dispersal rate) of this invasive anuran. The mechanism of impact plausibly involves reduced oxygen supply from infected lungs; if so, we expect to see that exercise will increase heartbeat rates more in infected toads than in uninfected conspecifics. Our data on 103 field-collected toads (53 of which contained lungworms) support this prediction. Exercise induced a greater increase in heartbeat rate in infected toads than in uninfected conspecifics, but no shift in oxygen saturation of the haemoglobin.