D-Fructose was isomerized to D-mannose using immobilized Agrobacterium radiobacter that produces a thermostable mannose isomerase. The cells were immobilized by adsorption on chitosan or by glutaraldehyde crosslinking in the presence of albumin. Optimum conditions for mannose isomerase activity were 60degreesC and pH 7.5. Continuous reaction at 55degreesC was achieved with immobilized cells packed in a column to produce D-mannose.