Trichinella spiralis larvae infect and develop within skeletal muscle cells causing major changes to their mechanical properties. The aim of this investigation was to determine the effects of T. spiralis on the power output and fatigue resistance of the mammalian diaphragm under conditions simulating in vivo operation and to relate these to respiratory performance. Infection with T. spiralis leads to major reductions in mechanical stress, work, power output and fatigue resistance. These changes are associated with the number of larvae present in the muscle and the duration of infection. However, the initial decline in mechanical performance occurs during the onset of infection when there are few larvae observed within the muscle cells, indicating that T. spiralis may affect the properties of muscle before encapsulation. This may correspond to the host's inflammatory response and the effects of larval excretory/secretory products. The decline in mechanical performance will have a profound effect on respiration both at rest and during exertion. This must influence the behaviour of the host and increase its chance of capture by predators, which is likely to benefit the parasite by facilitating its transmission.