Yeast cells immobilized by entrapment in an insolubilized gelatin gel have been investigated with regard to their pattern of growth and respiratory activity. Mass-transfer resistance offered by the matrix and growth of the entrapped cells determine a gradient of nutrients throughout the gel which is responsible for both a lower specific growth rate of immobilized cells with respect to that of free ones, and a heterogeneous biomass distribution, with progressively increased cellular density from the inside to the outside of the matrix. The spatial organization of the growing cells leads to the formation of a biofilm perfectly adherent to the surface of the matrix. Measurements of the oxygen-uptake rate of the immobilized cells, after having been released from the matrix by trypsin digestion, and confocal microscopy of cells stained with Rhodamine 123 demonstrate the occurrence of a gradient of respiratory activity throughout the immobilized culture.