A mathematical model for the startup period of a fixed-bed reactor with first-order reaction kinetics, axial dispersion and film mass transfer resistance was formulated and solved analytically using Laplace transform. The solution can be used to correlate or predict the reactant concentration profiles in the effluent of the reactor. The effects of both the Peclet number and Blot number on the effluent concentration profiles were studied. As an example, the approximate analytical solution was applied to experimental data of lactic acid fermentation in a fixed-bed fermenter with immobilized Lactobacillus delbrueckii cells.