Since the discovery of Legionella pneumophila, the causative agent of Legionnaires disease, two other groups of fastidious bacteria have been isolated which share many phenotypic characteristics with L. pneumophila [1-7] which have been referred to as Atypical Legionella-Like Organisms (ALLO) [2]. One group contains strains known as the Pittsburgh pneumonia agent (PPA) [6] and the Tatlock strain [8], for which the name Tatlockia micdadei has been proposed [4]. Members of the second group are serologically and genetically more diverse [2,3,7]. The name Fluoribacter [4] has been proposed for the latter group, and presently three species F. bozemanae, F. dumoffii and F. gormanii are recognised [4,7]. The taxonomic interrelationships of Legionella, Tatlockia and Fluoribacter are, however, equivocal, and additional criteria are required to tighten the generic and species descriptions. Ubiquinones (Coenzyme Q) are constituents of bacterial plasma membranes, and play an important role in electron transport and oxidative phosphorylation. Furthermore, recent studies have indicated that the structural variations exhibited by these compounds are of value in bacterial classification (see [9] for review). Ubiquinones, 2,3-dimethoxy-5-methyl-6-polyprenyl1,4-benzoquinones form a relatively large homologous series in which the length of the C-6 isoprenyl side-chain may vary from 1 to up to 12 isoprene units (Fig. 1). The major constituents of bacterial ubiquinones have been shown however to possess between 7 and 10 isoprene units (i.e. n = 7 to 10, Fig. 1) [9]. Recently, Finnerty et al. [10] reported Legionella pneumophila possessed ubiquinones as its sole respiratory quinones. Although the detailed structure of the ubiquinones of L. pneumophila was not determined, the authors noted that it possessed a side-chain containing more than 10 isoprene units [10]. This report is of considerable interest, as ubiquinone isoprenologues with more than 10 isoprene units have been found previously only in small amounts in a Pseudomonas sp. [11] and trace amounts in certain photosynthetic bacteria [ 12,13]. In this paper we report the structure of the ubiquinones of L. pneumophila. The opportunity was taken to also determine the ubiquinone content of other Legionella species, and representatives of the genera Fluoribacter and Tatlockia.