Modeling of gel-immobilized cell systems is presented and illustrated by Escherichia coli immobilized in thick agar membranes. The study describes in detail diffusion of the different species, the reactions transforming substrate into inhibiting product, and cell growth inside the membrane. Modeling, based on a free cell model previously established, gives the intramembrane substrate, product, and cell concentration profiles, together with the corresponding concentrations and pH in the external medium. The theoretical results are then confronted with a set of experiments run with two different membrane thicknesses. The agreement between theoretical and experimental results is good for all the characteristics tested that is, the intramembrane cell concentration profiles, the total intramembrane and external cell concentrations, the external substrate concentration, and pH. Nevertheless, the agreement requires the modification of one cell parameter : the maximum specific growth rate mu(m), signifying that immobilized cells are slightly different from free cells. The spatiotemporal evolutions of the different intramembrane concentration profiles thus enable us to understand the origin of the heterogeneous biomass distribution inside the membrane and to envisage a future study of the parameters driving this heterogeneity.