Abstract
SUMMARY: The primitive giant amoeba, Pelomyxa palustris, contains a population of endosymbiotic bacteria. The distinctive ultrastructure of one of the species of bacteria is described in detail. A particularly close relationship exists between these bacteria and the nuclei of Pelomyxa. The significance of some aspects of this relationship is discussed with reference to the endosymbiotic theory of the origin of mitochondria from bacterial precursors.