The kinetic parameters of soluble and immobilized beta-galactosidase of Kluyveromyces fragilis were determined using integrated reaction rate equations. Three reaction rate models were used to correlate experimental values by means of a multi-response non-linear computer program. The deviations of the fitted models to the data were compared using a residual sum of squares and F-test for nested models. The Michaelis-Menten equation predicted the best values with product inhibition by galactose and provided the most reasonable approach to describe the enzymic hydrolysis of lactose for low and high conversion in a range of initial concentrations from 2.5 to 15.0%.