Changes in cellular fatty acid composition during naphthalene degradation, at the concentrations of 0.5 g l(-1) or 1.0 g l(-1), by Pseudomonas sp. JS150 were investigated. In response to naphthalene exposure an increase in saturated/unsaturated ratio was observed. Additionally, the dynamic changes involved alterations in the contents of hydroxy, cyclopropane and branched fatty acids. Among the classes of fatty acids tested the most noticeable changes in the abundance of cyclopropane fatty acids were observed. Since day 4 of incubation these fatty acids were not dectected in bacterial cells growing on naphthalene. In contrast, markedly increased in the percentage of hydroxy fatty acids over time was observed. However, the proportions of saturated straight-chain and branched fatty acids did not change such significantly.