Growth of recombinant luminous E.coli immobilized in alginate beads revealed a novel observation: pellets with a low initial cell density may yield higher final density than those of high initial density. Observations of growth were combined with bioluminescence measurements that reflect the product of oxygen concentration and cell density distribution in a spherical gel pellet. To account for this behavior a model for growth of immobilized cells in the form of microcolonies was constructed and the behavior was simulated in a deterministic and stochastic (cellular automata) forms. Growth in the form of microcolonies was observed in this study as well as in other studies. Analytical and approximate solutions for zero or first-order kinetics predict that the final total cell density and the final thickness of active biomass layer vary with the initial cell density as a - 1/6 power. This exponent describes well the experimental results. Stochastic simulations revealed similar qualitative dependence. A pseudohomogeneous model of growth cannot account for this behavior.