A previously derived population model describing the average properties of hyphal elements in submerged cultu res of filamentous fungi was revised, and a term for the influence of spore germination on the average total hyphal length was added. The model was derived from a general balance for the distribution function for the hyphal elements. Based on experimental data and the derived model, simple kinetic expressions for spore germination, tip extension, branching, and hyphal break-up were set up. It is concluded that spore germination can be quantified by three parameters : (1) the time at which spore germination is initiated, (2) the time at which spore germination terminates, and (3) the fraction of viable spores in a spore suspension. The frequency of spore germination can be described with the B-distribution. For growth kinetics it is concluded that the branching frequency is closely correlated with the total hyphal length and that the average tip-extension rate can be described with saturation kinetics with respect to the hyphal length. Finally, the rate of fragmentation is linearly related to the energy input to the bioreactor, and related to the effective hyphal length.