Acinetobacter calcoaceticus was grown on ethanol in a chemostat as a model system for single-cell protein production. The substrate yield coefficient (Y(s), grams of biomass/gram of ethanol), protein yield coefficient (Y(p), grams of protein/gram of ethanol), and biomass composition were measured as a function of the specific growth rate. Nucleic acid, protein, Y(p), and Y(s) all increased at higher growth rates. Although protein content increased only 14% (from 53 to 67%), Y(p) almost doubled over the same range of growth rates. The increase in Y(p) was due to the higher protein content of the biomass and to higher values of Y(s). The higher values of Y(s) were attributed to maintenance metabolism, and the value of the maintenance coefficient was found to be 0.11 g of ethanol per g of cell per h. When A. calcoaceticus was cultivated under a phosphorus limitation protein content, Y(p) and Y(s) were lower than in carbon-limited cultures. It was concluded that a single-cell protein fermentation using A. calcoaceticus should be operated at a high growth rate under ethanol-limiting conditions in order to maximize both the protein content of the biomass and the amount of biomass and/or protein made from the substrate.