The effect of ammonium on growth of Candida apicola and on production of sophorose lipid was studied. Sophorose lipid production increased with increasing initial ammonium sulphate concentration. Both growth and product formation were strongly reduced at 73.6 mM ammonium. With 58.9 mM ammonium a microcrystalline sophorose lipid was formed. The ratio of the two isomers of the sophorose lipid, harbouring either omega- or omega-1 hydroxy fatty acid, was influenced by the initial concentration of ammonium. Both production kinetics, yields and profiles of the total cellular fatty acids express alterations with enlarged ammonium concentrations. These results suggest regulatory effects of ammonium on C. apicola and its sophorose lipid synthesis.