The adaptive responses of the bacterium Acinetobacter calcoaceticus to different aliphatic alcohols on the level of the membrane fatty acids were studied in detail. The toxicity of the aliphatic alcohols increased with an increasing hydrophobicity. As alcohols are known to increase the fluidity of the membrane they consequently should cause the same adaptive effect on membrane level. Yet, cells of A. calcoaceticus react completely different to the alcohols: in the presence of long-chained alcohols they increase their degree of saturation, while in the presence of short-chained alcohols they decrease the degree of saturation. So, there are no observable differences in the adaptive responses of bacteria with the so-called anaerobic pathway, like Escherichia coli and Pseudomonas putida, and the bacterium carrying the so-called aerobic pathway like A. calcoaceticus. These results strongly indicate a physico-chemical difference in the membrane effect of both the partitioning and localisation of the different alcohols into the membrane and the membrane adaptive responses of the bacteria to these effects.