The mycoparasite Sphaerellopsis filum (teleomorph Eudarluca caricis) was applied simultaneously with Melampsora larici-epitea on to willow leaf discs using eight concentrations of conidia. Inoculum densities were quantified and the numbers of uredinia of the rust, pycnidia and conidia of S. filum and rust spores produced per leaf disc were measured 13 d after inoculation (first assessment). Higher S. filum inoculum densities resulted in more rust uredinia being infected, but did not reduce the number of uredinia produced. The ratios of infected rust pustules: S. filum conidia applied were in a range of 0.25–0.31 when less than 20 S. filum spores were inoculated on to a leaf disc (0.95 cm2). Suppressive effects of S. filum on rust spore production were more obvious in the second assessment, carried out 23 d after inoculation. Inoculum densities of S. filum were significantly (P<0.001) correlated with the frequency of uredinia infected (% variance accounted for [VAF]=85.8), the number of S. filum pycnidia (%VAF=81.4), S. filum spores produced (%VAF=72.3) and rust spore production (%VAF=48.6). Rust spore production was significantly (P<0.001) negatively correlated with the frequency of uredinia infected (%VAF=51.1), the number of S. filum pycnidia (%VAF=42.0) and the number of S. filum spores produced (%VAF=40.6). The best correlation was found between the number of pycnidia and the number of S. filum spores produced (%VAF=88.8).