Direct exposure of the pentachlorophenol-mineralizing bacterium ' Sphingomonas' sp. UG30 to pentachlorophenol (up to 250 ppm), and sodium pentachlorophenolate (up to 1000 ppm), produced no membrane fluidity changes as indicated by fluorescence polarization and total cellular fatty acid analysis. No changes in the DNA, RNA or protein levels were observed in a starvation study involving inoculation of UG30 into sterilized river water in the presence of 50, 100 and 250 ppm NaPCP. Over the 90 d study the total (DAPI) cell counts were unchanged while the viable (drop plate) counts decreased. In these cultures the membrane fluidity decreased over the 16 d period of NaPCP degradation, suggesting increased membrane fluidity was due to nutrient starvation rather than the effects of NaPCP exposure. These findings show that while no membrane structural or macromolecular changes were observed after NaPCP or PCP exposure, UG30 retained its mineralization capability even after reductions in cell viability.