The isomerization of cis to trans unsaturated fatty acid in Pseudomonas putida, a mechanism of this bacterium to adapt its membrane to toxic environmental influences, was tested as an indicator for toxicity of 10 organic compounds (aromatics and aliphatic alcohols). A direct correlation was observed between the hydrophobicity of the compounds (logP), concentration dependent growth inhibition, and the trans/cis ratio of unsaturated fatty acids. The application of this system is discussed in terms of an indicator for toxicity and environmental stress particularly during bioremediation processes.