Conidia of Cladosporium sp. are frequently present in large numbers in collections of uredospores of Melampsora larici‐populina made in the field at Canberra in late autumn. The presence of these conidia considerably reduces the viability of the uredospores and consequently infection of leaves of Populus nigra var. italica under controlled conditions. Lysis of germinated and ungerminated uredospores under experimental conditions by the mycelium of Cladosporium sp. has been demonstrated. The potential for control of leaf rust of poplar by hyperparasitism is discussed. Copyright {\textcopyright} 1978, Wiley Blackwell. All rights reserved