Mollicutes have restricted metabolic activities, and catabolism is primarily associated with ATP generation rather than the synthesis of metabolic precursors for anabolic metabolism (1). Nevertheless, the pathways of energy substrate metabolism and the range of substrates used by mollicutes are diverse. In mycoplasmas, energy may be obtained by: the fermentation of sugars (via pyruvate) to lactate; the partial oxidation of organic acids, for example, of lactate or pyruvate, to acetate plus CO₂; and the metabolism of arginine, by the arginine dihydrolase pathway (2), to ornithine, NH₃ and CO₂. Individual Mycoplasma species may use one or any combination of these reactions to obtain energy, enabling the subdivision of the genus into major physiological groups (3). Within these groups, patterns and rates of substrate utilization have been shown to distinguish certain species and subspecific taxa (4–6), and may be applied to the biochemical characterization and identification of isolates. In addition, knowledge of the substrates used by mycoplasmas and other Mollicutes and their kinetics of utilization, may improve understanding of pathogenicity. Kinetic data are important in allowing assessment of the likely significance of substrate metabolism at the concentrations found in host tissues. The utilization of energy substrates at high rates may reduce substrate availability to host cells and result in the formation of toxic products, particularly hydrogen peroxide from carbohydrate metabolism (4).