In this work the quality and activity of immobilized Beneckea natriegens have been measured using Specific ATP (SATP) [mg (ATP) g(-1) (dry biomass)] and Specific Oxygen Uptake Rate (SOUR) [mg O-2 g(-1) (dry biomass) h(-1)]. The cells were grown in a 3 L Three Phase Air Lift Bioreactor (TPALB) and were immobilized on diatomaceous earth (silica) support particles; sterile conditions were employed during the experiment, with n-propanol as the sole carbon source. Two sets of experiments were performed, one with varying dilution rate and the other with varying inlet substrate concentration. The average SATP and SOUR of the immobilized biomass was 3-4 times lower than the values obtained for suspended Beneckea natriegens growing at its maximum growth rate. The suspended biomass present in the TPALB was generated primarily through attrition from the outer layer of the biofilm, and maintained higher levels of SATP and SOUR than the immobilized biomass. This result indicates that the immobilized biomass quality/activity is higher at the external layer of the biofilm. A structured model in which biomass is divided into two compartments, active and inert, was used to describe the experimental results. This model predicts the biomass quality/activity and substrate concentration distributions in the biofilm. These distributions were integrated to give overall values of SATP and SOUR for the immobilized biomass which compared favorably with experimental data. The primary implication of the results is that the location of immobilized biomass in the biofilm affects its biocatalytic activity, and should be taken into account when modeling immobilized biomass bioreactors.