A fluidized-bed reactor (FBR) was used to enrich an aerobic chlorophenol-degrading microbial culture. Long-term continuous-flow operation with low effluent concentrations selected oligotrophic microorganisms producing good-quality effluent for pentachlorophenol(PCP)-contaminated water. PCP biodegradation kinetics was studied using this FBR enrichment culture. The results from FBR batch experiments were modeled using a modified Haldane equation, which resulted in the following kinetic constants : q(max) = 0.41 mg PCP mg protein(-1) day(-1), K-s = 16 mu g 1(-1), K-i = 5.3 mg 1(-1), and n = 3.5. These results show that the culture has a high affinity for PCP but is also inhibited by relatively low PCP concentrations (above 1.1 mg PCP 1(-1)). This enrichment culture was maintained over 1 year of continuous-flow operation with PCP as the sole source of carbon and energy. During continuous-flow operation, effluent concentrations below 2 mu g 1(-1) were achieved at 268 min hydraulic retention time (t(HR)) and 2.5 mg PCP 1(-1) feed concentration. An increase in loading rate by decreasing t(HR) did not significantly deteriorate the effluent quality until a t(HR) decrease from 30 min to 21 min resulted in process failure. Recovery from process failure was slow. Decreasing the feed PCP concentration and increasing t(HR) resulted in an improved process recovery.