Abstract The effect of the growth rate on the macromolecular cell composition of the bacterium Acinetobacter calcoaceticus was investigated in carbon-limited chemostat culture. Only a minor change in the DNA and protein content was observed. Because of the improved cell yield at the higher growth rates due to the maintenance energy requirement of the cells, the protein yield increased markedly with increased growth rate, which is of significance for the production of single-cell protein (SCP). Increasing the growth rate caused the RNA content to increase more than two-fold. The RNA efficiency in protein synthesis also increased as a function of the growth rate, supporting the hypothesis that ribosomes are saturable sites of protein synthesis.