1. Concentric bodies have been detected in the mycobionts of at least forty-three species of lichens, and their presence in two non-lichenized fungi is reported here for the first time. The structure of the bodies appears to be closely similar in all species. 2. A detailed examination of the concentric bodies in electron micrographs of thin sections of Peltigera aphthosa Willd, indicates that the bodies are basically isodiametric in organization. The designation “concentric bodies”, a translation of the term “konzentrische Korper” first used by Peveling (1969a), is therefore preferred to the original name “ellipsoidal bodies” (Brown and Wilson, 1968). 3. Each concentric body is composed of two zones of osmiophilic material surrounding an electron-transparent core. The inner zone is limited internally by a membrane-like boundary. The outer zone has a variable appearance, but often contains radially arranged stainable structures which appear as lamellae in tangential sections. Concentric bodies may collapse under stress into discs with elimination of the core but without apparent rupture of the surrounding material. It is concluded that the bodies are empty or filled with gas or liquid. 4. The bodies occur singly or in clusters and are frequently surrounded by electron-transparent haloes. Clusters of bodies lie within a distinctive matrix which is almost invariably associated with the cell nucleus. 5. Concentric bodies have been found in all types of hyphae in the lichen thallus but not in the asci or ascospores. 6. Interpretations of previous workers are discussed in relation to the present findings, and hypotheses as to the nature of the concentric bodies are put forward. 7. No information is as yet available concerning the origin, development and functions of the concentric bodies, and the question of their possible significance for the lichenized condition remains open.