Glycogen storage in host tissue and symbiotic bacteria in the anterior trophosome of the vestimentiferan tubeworm Riftia pachyptila Jones, 1981, was investigated using transmission electron microscope and stereological methods. The relative glycogen content (RGC) of each partner was calculated from the percentage of host and bacterial cytoplasm area taken up by glycogen and from the percentage area that host and symbionts occupy within a trophosome lobule section. Our results show that host and symbionts contribute equally to the total glycogen content of the trophosome; this ratio remains similar for up to 40 h of hypoxia. Furthermore, there is a glycogen gradient in the lobule. In both symbiotic partners, the glycogen content increased from the lobule center toward the periphery, implying different metabolic activities of host cells and bacteria depending on their location within a trophosome lobule.