A severe epidemic of Endothia gyrosa in a plantation of Eucalyptus nitens at Tewkesbury in Tasmania prompted a comparison of the pathogenicity of isolates from this site with that of isolates from elsewhere in Tasmania and mainland Australia. Sixteen isolates were artificially inoculated on 12-month-old seedlings of two major plantation species, E. nitens and E. globulus. The majority of isolates produced lesions on both host species that were significantly different in size to those in non-inoculated seedlings and were not callused-over at 7 months after inoculation. Seedling mortality was negligible. Certain isolates originating from various regions in mainland Australia and from the Tewkesbury site appeared more pathogenic although differences in lesion size between isolates was not always significant. Only one out of seven isolates from Tewkesbury demonstrated significantly higher levels of pathogenicity than all other isolates although there was a trend for isolates from this site to cause greater lesions in size. It is, however, unlikely that the epidemic caused by E. gyrosa at Tewkesbury, even though of far greater impact than previously observed on plantations in Australia, is solely the consequence of more pathogenic strains. This conclusion does not exclude the possibility that more pathogenic strains of an opportunist pathogen such as E. gyrosa could have played some determinant role in epidemic development.