A reliable, rapid and low-cost screening bioassay, based on measuring lesion sizes, rotten tissue weight and tissue colonization 1 week after the injection of a conidial suspension into the stem scars of mature green tomato fruits, was developed to assess the aggressiveness of Colletotrichum coccodes isolates. This protocol was compared with inoculation of either potato plantlets from in vitro multiplication, or ripe tomato fruits, using C. coccodes isolates from potato and from tomato. Aggressiveness to mature green tomato fruits was scored by measuring lesion size and weight at the scar end of the fruit, while colonization was measured in samples taken from the blossom end. Values of all three disease parameters were significantly higher (ca. 3 times) with aggressive isolates of the pathogen than with less aggressive ones. High correlation levels among these three parameters were obtained. Also, the aggressiveness to mature green tomatoes was highly correlated with the aggressiveness to potato tissue culture plantlets. It was concluded that lesion size in inoculated tomatoes can be used as a sole measure for estimating the aggressiveness of C. coccodes isolates to either tomato or potato.