We confirmed, with the electron microscope, that Trichinella larvae feed directly on the host tissue during their initial development. They suck, by means of a stylet acting as a piston, all components of the muscle cell which develops into a nurse cell, into their oral cavity. The stylet is controlled by a muscle retractor. At day 10 p.i., fragments of myofilaments and glycogen are present in the oesophagus of the larva of T. nativa, mitochondria occur in it at day 20 p.i. During the larval growth of T. pseudospiralis, remnants of a contractile material are present in the digestive system. The composition of the food ingested by the larva is in agreement with the development of changes in the muscle fibre. Initial changes in the fibre caused by the two Trichinella species are comparable, later changes are not. T. pseudospiralis influences the character of changes by its uninterrupted movement inside the fibre. From days 50-90 p.i. with T. pseudospiralis, a focally distributed sarcotubular system proliferates irregularly in different sites of the fibre. This is in contrast to a heavy proliferation of tubules inside a concentric membranous system around the larva of T. nativa becoming enclosed in a capsule. The larva which has ceased to feed on the host tissue at this time, feeds apparently on low-molecular substances of the membranous system transported from the surrounding vascular system which has increased in quantity in the endomysium. In the later course of the infection, the enzymatic activity of the AIP and the SDH increases in the vicinity of the larva of T. nativa, i.e., in the sarcoplasm inside the capsule.(ABSTRACT TRUNCATED AT 250 WORDS)