Pterotermes occidentis is a large, obscure, very primitive dry wood termite limited to the Sonoran desert of North America. Its development, caste system and behavior are discussed in relation to aspects of its ecology. What appear to be single wood-digesting individual termites are in fact insect hosts which harbor extensive microbial communities estimated to contain more than 40 interacting species. All healthy, wood-eating Pterotermes contain these densely populated communities of highly motile symbionts suggesting that species diversity is necessary for termite survival. A morphological catalogue of the major hindgut microbes is presented here. From 10(7) to 10(11) bacteria and from 10(3) to 10(6) protists/ml of hindgut fluid are found in an average nymph or larva. The species of four wood-ingesting mastigotes (genera Trichonympha, Metadevescovina, Tricercomitus and Microrhopalodina) and nearly 30 morphologically distinct types of bacteria are described. The description of this morphological diversity observed with light, scanning and transmission electron microscopy is a prerequisite to functional analyses of the symbioses. Since Pterotermes is easily maintained in the laboratory and is so large it is hoped that it will provide a useful model for symbiosis research.