Batch cultures of Geotrichum candidum and Penicillium camembertii were carried out on peptones as carbon and nitrogen source and in the presence of lactate as a second carbon source. Unless growth ceased, carbon and nitrogen yields remained constants, except yields involving lactate consumption by G. candidum, since this fungus preferentially metabolized peptones as a carbon source. For both fungi, nearly 40% of the available carbon was metabolized for cellular biosynthesis and the remainder (about 60%) as carbon dioxide, for the energy supply of both biosynthesis and viable cell maintenance. Moreover, in relation to their carbon content, amino acids contain excess nitrogen, which was released as ammonium. From all these, the yields of ammonium nitrogen on cellular nitrogen were in all cases higher than 1, and were especially high when the medium contained only peptones as a carbon source, 4.4 and 5.7 for G. candidum and R camembertii respectively. Indeed, in this case, the excess nitrogen was especially pronounced. (C) 2003 Elsevier Ltd. All rights reserved.