In this study, microbial production of rennin, a milk-clotting enzyme, from a commercial strain of Mucor miehei NRRL 3420 has been investigated in a continuously fed femtenter for prolonged times. The spherical film-type growth of the culture has been accomplished in the fermenter and the effects of medium pH, mixing and dilution rates, and feed D-glucose concentration on milk-clotting activity have been elaborated. In the fermenter, optimum operational parameters have been determined as 400 rpm 0.125 day(-1), and 7.5 g l(-1) for mixing rare, dilution rate, and feed D-glucose concentration respectively. Under these conditions, the fermenter operated 575 h continuously producing 1.24 IU ml(-1) maximum milk-cloning activity without concentration. ra the fermenter sample at maximum milk cloning activity, the R factor and specific milk-clotting activity were determined as 1.55 X 10(-3) IU PU-1 and 5.28 IU mg(-1) medium protein, ＇respectively, denoting competitive characteristics of a commercial rennet after concentration.