Pentachlorophenol (PCP) degradation by Flavobacterium sp. cells was investigated. The initial PCP concentration affected the survival rate of Flavobacterium cells and the lag in PCP degradation. Simple models which assume that only a fraction of all the inoculated cells i s resistant to the level of PCP encountered could account for the results observed. This fraction is assumed to grow exponentially on and degrade PCP while a first order kinetics represents adequately the death kinetics of the sensitive fraction of cells. A second model incorporates the existence of a lag.