A theoretical discussion on pellet formation of Aspergillus niger based on shear rate or average velocity gradient (G value) was made for both cases of a reactor with paddle and a rotationally shaken flask. The average diameter of pellets (d) and the number of pellets formed per unit volume (n) are linearly related to the G value below 4001/s on a logarithmic plot, and a regression equation, n = (1.37 x 10(-5)).d(-3.25), was obtained regardless of the agitation and inoculation conditions. Under non-turbulent agitation conditions, the average number of spores forming a single pellet was found to be approximately proportional to the two-thirds power of the concentration of the spore inoculated. These exponents are very close to the theoretical one. Consequently it is concluded that G value is a representative index for the evaluation of pellet formation capability, verifying the validity of the proposed theory.