Pythium radiosum Paul has been found to be an aggressive mycoparasite of Botrytis cinerea which causes grey mould disease on the grape-vine. The mycoparasitic fungus enters the host mycelium, coagulates its protoplasm, empties its contents and finally comes out producing numerous ramifications and sexual structures. When the infected mycelium of B. cinerea is applied to the leaves of the grape-vine, the characteristic grey mould symptoms fail to appear. Since P. radiosum causes no harm to the grape-vine, it can be used as a biological control agent against B. cinerea. A brief account of the mycoparasite and its antagonism towards B. cinerea is discussed here.