Longitudinal dispersion in fluidized bed reactors was studied using pulse-response techniques for both clean and anaerobic-biofilm coated media. A large number of experimental data on the longitudinal dispersion and biofilm growth in the anaerobic fluidized bed reactors (AFBRs) were investigated. Some correlations applicable to fluidized beds were obtained far both the hydraulic retention time and the biomass concentration versus the ratio Pe/Re. The biomass concentration tends to zero for a critical retention time in AFBRs. The biological growth in the bed causes an increase of Pe number. Copyright (C) 1996 IAWQ.