Beneckea natriegens growing bath in suspension and immobilised on diatomaceous earth (silica) support particles, in a 3 dm three phase air lift bioreactor (TPALB), was employed to demonstrate the relationship between substrate loading rate and immobilised biomass loading at steady state. Sterile conditions were maintained during the experiments, while the hydraulic conditions and the inlet flowrate remained constant. The experiment ran for 12 days, whilst the inlet substrate (n-propanol) concentration was varied both upwards and downwards between 250 and 750 mg dm(-3). The bulk liquid substrate concentration and the immobilised biomass loading were found to vary between 6-120 mg dm(-3) and 71-159 mg (dry biomass) g(-1) (silica), respectively. The experimental results demonstrated that there is a positive relationship between substrate loading and immobilised biomass loading. A mathematical model is proposed to calculate biofilm thickness and bulk liquid substrate concentration (for steady state) using the inlet flowrate and the inlet substrate concentration as input parameters. The model was able to predict both biofilm thickness and immobilised biomass loading for the lower substrate loading rates applied.