A strain of Rhizopus stolonifer produced a high activity of extracellular DNAase when grown on YPG (yeast extract peptone glucose) medium. The source of peptone had a marked effect on the enzyme yield and only one peptone (i.e. from Sarabhai M. Chemicals Ltd, India) supported enzyme production. Maximum enzyme activity (88 U/ml) was obtained after 4 days' growth under submerged conditions in YPG medium containing 100 μM Mn(2+), Co(2+) or Mg(2+), and glucose as the sole carbon source. The unpurified enzyme was optimally active at pH 7.5 and 45°C. It had a higher activity with sonicated and heat-denatured DNA than native DNA.