Abstract
Summary: Prochloron cells from a variety of ascidian hosts and from sites throughout the tropical Pacific and Indian Oceans have been studied by optical and electron microscopy. With a few exceptions, they can be placed into three typological groups which correlate closely with the type of host–symbiont relationship. This grouping is also compatible with the limited biochemical taxonomic data so far published on Prochloron. Some inferences can be drawn about the evolution of Prochloron and its symbiosis with ascidians.