Interactions between the mycoparasite Cladosporium tenuissimum and the bean rust Uromyces appendiculatus were studied through light and electron microscopy in vitro at the host-parasite interface. Urediniospore germination decreased on contact with ungerminated C. tenuissimum conidia, possibly due to antibiosis mechanisms. C. tenuissimum grew towards the bean rust spores and coiled around their germ tubes. Penetration of the urediniospores occurred either enzymatically and/or mechanically, through appressorium or infection cushion structures, from which a thin penetrating hypha was generated. Enzyme production by the mycoparasite was suggested by the loosening of the matricial components of the spore wall, which sometimes left chitin fibrils visible. Mycoparasite hyphae grew within the host spore, emptied its content, and emerged profusely forming conidiophores and conidia. C. tenuissimum was able to grow on media containing laminarin, suggesting the ability of producing glucanases, but not when chitin was used as the sole carbon source. Conidia that had been grown on a sugar-rich medium, filtered, and extracted with organic solvents, were found to contain cladosporol and related compounds. Complete control of the bean rust disease was achieved by application of C. tenuissimum culture filtrates but not by conidial suspensions. This is the first report of parasitism by C. tenuissimum on U. appendiculatus. These investigations provide additional observations on a genus besides Melampsora and Cronartium from which this fungus has been isolated and tested to date. The possible role of environmental factors for the exploitation of this organism as a biocontrol agent is also mentioned.