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This study explored the suitability of using the trans/cisratio of unsaturated fatty acids as an indicator of the acute toxicity of membrane active hazardous chemicals. The conversion of cisinto transfatty acids in Pseudomonas putidaNCTC 10936 in response to 4-chlorophenol and temperature changes was compared with the results from another kind of toxicity test using the same organism, based on the sensitivity of its xylose oxidation-driven ATP synthesis to uncoupling. The response of both indicators is believed to be largely due to changes in the fluidity of the cytoplasmic membrane. However, the electron transport phosphorylation reacted faster and more sensitively to the fluidizing effect of 4-chlorophenol than the isomerization of unsaturated fatty acids. Therefore, measuring the trans/cisratio does not provide as good early warning signals of acute toxicity as monitoring the response of the electron transport phosphorylation. If used as an indicator of chemostress, with Pseudomonasspecies as test organisms, the ratio should only be used in conjunction with other parameters reflecting the energetic state of the cells.