A mathematical model for a hollow-fiber beaker fermenter was pressented to investigate time-dependent behaviour of the fermenter. Tube side of the fermenter was modelled as n-mixed tanks and the shell side was segregated from the tube side by membrane. To compare numerical simulation with experimental data, Saccharomyces cerevisiae was cultured in the shell side. When the initial substrate concentration in the shell side was low, history of outlet tube-side substrate concentration has a complex pattern instead of monotonous profile. Membrane permeability was observed to decrease rapidly with the growth of cells.