An automated system for the monitoring of fermentations of filamentous fungi is described. The system is based on the on-line combination of ultrafiltration, for the removal of cellular and macromolecular material from the fermentation broth, and column liquid chromatography analysis of the filtrate. The performance of one hollow-fibre and two planar ultrafiltration modules is evaluated. The maximum sampling frequency as well as prevention from clogging by mycelium is strongly dependent on the construction of the module, best results being obtained with a planar membrane and a relatively high flow rate (150 ml/min) of the broth through a single, wide-bore (3mm) flow channel in the module. The method is used for the study of metabolic and regulatory processes of two different Aspergillus niger strains on multiple carbon sources. The selected system can be applied for at least 70 h without any negative effect on either the fermentation or the analytical system. Through an analysis frequency of once per hour detailed information regarding consumption and production of nine different compounds could be obtained.