The ability of intracellular marine zoochlorellae (Chlorophyceae) to provide their host with photosynthetically fixed carbon in vivo or with algal by-products via hetrophagy or autophagy is examined in the coelenterate A nthopleura xanthogrammica. Algae do fix 14CO2 within the gastrodermal tissue layer of the host and the level of 14C present within the gastrodermis is dependent upon the nutritional state of the host. The various techniques used to detect the percent light-fixed 14C present within the tissues of the host result in values ranging from less than zero to 3.9%, independent of the nutritional state of the host, indicating that relatively small amounts of soluble fixed carbon are translocatedto, the host's tissues. Acid phosphatase reaction product is absent within vacuoles containing algae; cellular debris which co-occurs within these same vacuoles is also free of reaction product, suggesting that the presence of an alga within a vacuole inhibits (in an unknown manner) the cellular digestive processes of the host. Further, reaction product is restricted to the exterior of the cell wall following expulsion of algae into the host's coelenteron, suggesting that the cell wall is resistant tolysosomal hydrolase activity. The distribution of acid phosphatase reaction product in situ suggests that little in the way of algal by-products would be made available to the host by the process of heterophagy or autophagy.