The ability of five yeast strains (locally isolated) to ferment xylose to xylitol were screened using a corn cobs hydrolysate. Candida tropicalis proved to be the best producer. The effects of culture conditions, namely initial pH, nitrogen source and yeast extract concentration on xylitol production were evaluated. The conditions for batch production of xylitol, using C. tropicalis immobilized growing yeast cells in hydrogel copolymer carrier have been optimized. The effects of supplementation of fermentation medium with different concentrations of metabolic inhibitors (monofluoroacetic acid or monochloroacetic acid) and methanol as an activity modifier, on xylitol production by immobilized cells were studied. In repeated batch fermentation (4 days for each run), the immobilized cells retained their ability to produce the superior xylitol yield (48.5 gL") in presence of 10 mg % of monofluoroacetic acid and 1.5 ml % methanol supplemented together to fermentation medium. The results also demonstrated that during about 5 weeks of repeated batch under optimized conditions, the final xylitol concentrations decreased I y in nine consecutive runs.