Cells of the yeast Candida guilliermondii have been immobilized by adsorption onto sawdust and used in a bubble-column reactor for the continuous production of citric acid. At a dilution rate of 0.21 h(-1) in a nitrogen-limited medium containing glucose, a reactor productivity of 0.24 g l(-1) h(-1) has been achieved which is twice that observed in a batch fermenter culture using freely suspended cells. The corresponding specific production rate was 0.024 g citrate g(-1) biomass h(-1) while the yield was 0.1 g citrate g(-1) glucose utilized These latter values were lower than those observed using freely suspended cells, indicating that further improvements can be made to the operation of the reactor. In comparison with literature reports describing other cell immobilization techniques, adsorption onto sawdust allows similar reactor productivities while being cheap and permitting simple immobilization and reactor operation.