The production of the polysaccharide pullulan by immobilized cells of the fungus Aureobasidium pullulans ATCC 201253 was studied using aerated column bioreactors. The reduced pigmentation fungal cells were grown in a phosphate-buffered minimal medium and then were adsorbed on the ion-exchange resin triethylaminoethyl-cellulose at a pH of 2.0, 6.0 and 7.5. Using column bioreactors, polysaccharide synthesis by the immobilized cells adsorbed at each pH was followed over a period of 14 days. The polysaccharide in the culture medium eluted from each column was precipitated by ethanol and its level was determined gravimetrically. Polysaccharide production was lowest for the cells immobilized on triethylaminoethyl-cellulose at pH 2.0 during the initial production cycle but increased during the second cycle. The cells adsorbed on triethylaminoethyl-cellulose at pH 7.5 produced the highest pullulan levels during the initial production cycle but produced very little polysaccharide during the second cycle. The cells immobilized at pH 6.0 produced polysaccharide during both cycles but enhanced polysaccharide production was observed during the second cycle. Despite polysaccharide production by the pH 7.5-adsorbed cells being higher during the initial cycle, it appeared that the cells adsorbed at pH 6.0 could be more effectively used for the semicontinuous production of pullulan. In conclusion,  column bioreactor polysaccharide production was affected by the pH  used  to  immobilize the fungal cells on the ion-exchange resin