Cell growth and production of nystatin by Streptomyces noursei (ATCC 11455) were investigated on the three different nitrogen sources, ammonium sulphate, ammonium nitrate and sodium nitrate. S. noursei was able to utilise all of the three tested nitrogen sources for the growth and production of nystatin. High ammonium concentration had a negative effect on production of nystatin when phosphate and glucose was in excess. There was an increased production of nystatin when the cultures became ammonium limited. Cultivation with sodium nitrate as the nitrogen source resulted in a prolonged lag-phase for growth and about 50% lower final nystatin titres compared with cultures grown on nitrogen sources containing ammonium. Nystatin production was shown to be related to the specific growth rate, its production was increased at decreasing specific growth rates.