The growth and biomass loss pattern for immobilized cells growing on diatomaceous earth particles in a three-phase airlift bioreactor (TPALB) is studied using S-35 as a radiolabelled tracer. A monoculture of Beneckea natriegens was grown immobilized on support particles in a 3 L TPALB. Sterile conditions were maintained during the experiments, and n-propanol was used as the sole carbon source. After the system reached steady state, the unlabelled sulfate (SO42-) in the feed tank was substituted by a radioactive grade ((SO42-)-S-35), and the assimilation of S-35 in the immobilized and in the suspended cells was monitored. The results indicated that the growth rate of immobilized biomass was not uniform throughout the biofilm. More specifically, it was concluded that cells growing closer to the external biofilm layer exhibit a higher growth rate, and that biomass loss from the biofilm was through attrition at the outer biofilm surface rather than by sloughing off of whole sections of biofilm. (c) 2006 Society of Chemical Industry.