Rotary biofilm contactor (RBC) is a reactor consisting of plastic discs that act as supports for micro-organisms. The discs are mounted on a horizontal shaft and placed in a medium-containing vessel. During nitrogen-rich growth phase, mycelia of Rhizopus oryzae ATCC 20344 grew on and around the discs and formed the "biofilm" of self-immobilized cells on the surface of the plastic discs. During the fermentation phase, the discs are slowly rotated, and the biofilms are exposed to the medium and the air space, alternately. With RBC, in the presence of CaCO3,Rhizopus biofilm consumes glucose and produces fumaric acid with a volumetric productivity of 3.78 g/L/h within 24 h. The volumetric productivity is about threefolds higher with RBC than with a stirred-tank fermenter with CaCO3. Furthermore, the duration of fermentation is one-third of the stirred-tank system. The immobilized biofilm is active for over a 2-wk period with repetitive use without loss of activity.