In previous papers, it was shown that eukaryotic microbial systems can be encapsulated in polyelectrolyte complexes (PEG) prepared from sodium cellulose sulfate and poly(dimethyldiallylammonium chloride) with maintainance of vitality. In the present study, prokaryotic cells were successfully encapsulated in these PEG. Serratia marcescens B345 (IMET 11312) was chosen as a model organism. This strain converts gluconic acid to 2-ketogluconic acid. Since the 2-ketogluconic acid produced has very strong complexing properties, the number of applicable immobilization methods is restricted Due to the high stability of PEC towards complexing agents these problems can be overcome by the described method. As already described in previous papers, a preimmobilization of cells in a PEC coprecipitate prior to capsule formation proved to be advantageous also for encapsulation of bacilli. The mean productivity of the encapsulated S. marcescens cells was 1-4.4 g l(-1) h(-1) in comparison to 5 g l(-1) h(-1) for free cells. The productivity Mns highly dependent on the flow rare of the reactor. The encapsulated cells were used for 1,200 h in a continuous biotransformation process for the production of 2-ketogluconic acid.