The germination of spores is the initial stage of fermentation processes, in which filamentous fungi are involved. The qualitative and quantitative description of this process is very important due to the fact that the final productivity of a primary or secondary fungal metabolite is usually strongly influenced by the quality of spores and the kinetics of their germination. Digital analysis of the images of spores and chemical analysis of the germination medium have been applied in this paper to quantitatively describe the spore germination kinetics for the filamentous fungus Aspergillus niger, which accumulates citric acid. The performed experiments provided data for the formulation of a kinetic model for the process of spore germination. Three pools of substrates: internal storage components, N-source, and C-source were taken into account. The biomass content was indirectly determined as a mean projected area of hyphae. The model was further validated for A. niger growth on different substrates and used for estimation of biomass concentration, expressed as dry weight, in the system. (c) 2005 Elsevier Ltd. All rights reserved.