The dynamic response of an immobilized-cell, fluidized-bed reactor (ICFBR) to step changes in phenol loading was investigated at 10degreesC for a pure culture of Pseudomonas putida Q5, a psychrotrophic bacterium. A novel dynamic model was developed and tested to simulate the response of all four key process variables: the bulk phenol concentration, the suspended biomass concentration, the concentration profile of the substrate in the biofilm and the biofilm thickness. Accurate model predictions required the use of kinetics, determined using cells which were not acclimated to the post-shock reactor conditions ('unacclimated' cells) and the implementation of a specific-growth-rate suppression factor to account for the unbalanced growth situations experienced during the transient response periods.