The effects of three aromatic compounds, p-cresol, 2,4-dichlorophenol (DCP), and 2,4,6-trichlorophenoI (TCP), on cell growth and pentachlorophenol (PCP) degradation by a Flavobacterium species were investigated. While p-cresol was not degraded by this bacterium, DCP and TCP were simultaneously degraded with PCP. Both DCP and TCP lowered cell growth and PCP degradation rate. Cell growth was modeled by cell death, because p-cresol, DCP, and TCP were toxic to the organism. A new model was used to predict cell death rate in a mixture of two toxic compounds from the cell death kinetics for each individual compound. PCP degradation rates were modeled by conventional inhibition models, but only over a small concentration range for the secondary toxic compound. However, a new empirical model described PCP degradation over a wider concentration range of the secondary toxic compound.