Geotrichum candidum growth on ammonium and leucine as nitrogen sources and glucose as a carbon source was examined. A clear preference of G. candidum for ammonium over leucine as a nitrogen source was shown. Indeed, ammonium was completely exhausted at the end of exponential growth after less than 35 hrs of culture; in contrast only 5% of leucine was concomitantly assimilated. Growth continued at slower rates on glucose and leucine as carbon and nitrogen sources respectively, and at the end of culture (185 hrs), leucine was completely exhausted.