The muscle fibres of mice containing the infective-stage larvae of the nematode Trichinella spiralis have been studied by means of the freeze-fracturing technique. The larva lies in what appears to be a fluid-filled cavity within the cytoplasm of an altered muscle fibre. There is no membrane separating the cytoplasm of the nurse cell from the cavity surrounding the larva which is therefore truly intracellular, unlike many parasites that reside within a membrane-lined parasitophorous vacuole within the host cell. This altered muscle fibre, known as a nurse cell, lacks myofilaments but does contain extensive cisternae of endoplasmic reticulum; membrane-bound vesicles are budded off from the endoplasmic reticulum and traverse the cytoplasm towards the cavity containing the nematode where they apparently pass into the cavity. It is suggested that the contents of these vesicles are used to sustain the nematode. Attention is drawn to the similarity to giant cells that have been induced by the plant-parasitic nematode Meloidogyne in the roots of host plants and which sustain the nematode. The conversion of the muscle fibre into a nurse cell is probably brought about by the presence of a metabolic sink, the larval nematode, within the cell. This take-over of the control of a metazoan cell by another metazoan organism is most unusual and warrants further study.