Bioconversion of glucose to gluconic acid at low pH using Aspergillus niger immobilized on cellulose fabric was investigated. Glucose solution (100 g l(-1)) was made to how through capillaries of a vertical fabric support, used for immobilization and is oxidized to gluconic acid at the interface. Conditions of temperature, humidity, airflow, and glucose feed rate have been optimized. The system could be run continuously for a period of 61 days utilizing the entire available glucose. The emerging broth contained a product concentration of 120-140 g l(-1) of gluconic acid which is higher than the expected [maximum of 109 g gluconic acid 100 g glucose(-1)] as a result of evaporative concentration during the downward flow.