Four strains of mesophilic lactic acid bacteria were separately or coentrapped in K-carrageenan/locust bean gum gel beads and used for continuous prefermentation of UHT skim milk in a stirred-tank bioreactor. Lactic acid and cell productivities of the immobilized cell bioreactor were particularly high and remarkably stable. during eight weeks of continuous milk prefermentation (about 18 g h(-1)l(-1) of lactic acid and 4.9 x 10(12) CFU h(-1) l(-1), respectively, but important variations of the bacterial populations in prefermented milk and gel beads occurred in any case (co- or separate entrapment). The strain Lactococcus lactis subsp. lactis biovar diacetylactis CDI1 became dominant, accounting for approx. 90% (released cells) and 78% (immobilized cells) of the total population. Microscopic observations of sections of gels beads showed a progressive destructing of the bend surface with rupture and release of entrapped viable cells from peripheral cavities of the gel. It is believed that these cavities close again after releasing all or part of their cell content, entrapping the different strains of the mixed culture and initiating a new colonization step and a cross-contamination of the beads. On the other hand, experimentations over seven-week periods with pasteurized milk showed the high resistance of the immobilized cell bioreactor to psychrotrophic contamination.