Microorganisms capable of degrading di-n-butyl phthalate (DBP) were immobilized in calcium alginate gel beads. The radial distribution and bioactivity of the microbial cells within the gel beads were investigated. The dissolution of calcium alginate gel beads in sodium citrate (20 g/l) showed that the decrease in bead diameter was linear. The distribution of biomass inside the gel beads was heterogeneous, with progressively increased cellular density from the inside to the outside of the matrix. Measurements of oxygen-uptake rate of the immobilized cells, after release from the matrix by sodium citrate demonstrated the occurrence of a gradient of respiratory activity throughout the immobilized culture. (C) 2000 Elsevier Science Ltd. All rights reserved.