Mycelia from a culture of Rhizopus spp, isolated from soil, were immobilized on cellulose microfibrils support and used for bioconversion of sucrose to dextran. Both batch and continuous modes of operation were investigated. For immobilized mycelia in batch (shake flask) fermentation, dextran yield and specific rate of production were 14 g l(-1) and 0.4 g g(-1) h as compared to 11 g l(-1) and 0.28 g g(-l) h for free mycelia. In continuous fermentation, sucrose solution (45 g l(-1)) flows through the capillaries of the fabric support on which mycelia were pregrown, and is bioconverted at the interface to dextran. The system could be run continuously for a period of 25 days and the emerging broth showed higher yields of dextran presumably as a result of higher interface interactions between substrate and mycelia. The eluate contained 22 g l(-1) of dextran, which was higher than expected as a result of evaporative concentration. Lignin in the medium is seen to be beneficial to both the final yield and specific rate of production. The effect of water activity on the bioconversion is described.