Isolates of Sclerotinia sclerotiorum differ significantly in aggressiveness on sunflower. In total, 120 isolates were collected from head and basal stem rots of sunflower in two locations in south-east Queensland, Australia. The inoculation of sunflower stems with mycelial plugs and the measurement of lesion development were used to compare aggressiveness between isolates. Rank ordering of isolates indicated differences, with a few isolates displaying high and low aggressive abilities. Differences in aggressiveness were more indicative of a continuous variation rather than discrete groups. Isolate aggressiveness did not correspond to the location of collection nor to the mode of pathogen reproduction or infection from which they were derived. Statistically significant differences between the isolates differed according to the statistical test employed, as different multiple comparison procedures had a greater influence in interpretation of aggressiveness than the isolates themselves. Results indicate individual isolates do not maintain discrete aggressive abilities across the multiple comparison procedures, with the exception of weakly aggressive isolates.