Introduction of excretory/secretory (ES) products of both infective-stage and newborn larvae of Trichinella spiralis into cultures of primary rat myocytes elicited morphological and structural changes in the myotubes. They appeared more granular, thinner, and failed to form networks. The most prominent lesion was the formation of 'nodular' structures, each bearing an enlarged nucleus, along the myotubes. Each node contained numerous cavities enclosed by an intact sarcolemma. Co-culture of myocytes with newborn larvae also elicited nodular formation but each node contained a large central cavity encircled by smaller ones. An immunocytolocalization study using IFAT and laser confocal microscopy showed the presence of parasitic epitopes inside the nodes. However, ES products from adult worms did not affect the myotubes.