Since continuous monitoring of mycelial growth in liquid media is ;still difficult, indirect ways of calculating on-line the total biomass concentration are of great interest. Growth of fungi like G. candidum and P. camemberti (widely used in cheese ripening) in liquid media may induce large pH changes. When nitrogen is supplied by glutamic acid (also able to supply carbon), an alkalinization of the medium is recorded, irrespective of the presence of an extra carbon source; when nitrogen is provided by lysine (unable to supply carbon) supplemented by glucose, an acidification is observed. Depending on the amino acid used the pH variation is linearly correlated with growth (glutamic acid) or not (lysine). A simple model predicts that the concentration of proton transferred vary in proportion to that of the total biomass: the former variable may be easily derived from the pH history and the buffer capacity of the medium. For all combinations of nitrogen and carbon sources tested, the model has been experimentally validated for both microorganisms during the main growth period: following a preliminary calibration, fungal growth may be monitored on-line by pH measurements.