Acrylamide production from acrylonitrile has been carried out in a specially designed fluidized bed reactor using immobilized cells of Brevibacterium sp. A4. The cells were entrapped in modified barium alginate beads. The optimum conditions with regard to catalyst stability were determined as: pH, 7.5; temperature, 5 °C; maximum acrylamide concentration, 60 g l−1. There was no diffusional limitation due to the gel. On line determinations of the kinetic constants were performed. A high value of the maximum hydration rate was found but, the Michaelis-Menten constant for acrylonitrile, Km, and the inhibition constant for acrylamide, Ki, appeared to be higher than the values obtained for free enzyme. The volumetric productivity of the reactor varied with both the substrate concentration and the feed rate.