A wide range of pathogenic variation was observed among the five isolates of Cylindrocladium quinqueseptatum leaf blight (CLB) which could be distinguished as different physiologic strains on 11 differential provenances of Eucalyptus. Susceptibility ranking of different provenances to five isolates also differed significantly indicating differential interaction between isolates and provenances. Analysis of variance showed that CLB severity of a provenance is mainly governed by the genetically different isolates and also that the provenances have closer genetical relationship. The results provide the first evidence for the existence of physiologic strains in C. quinqueseptatum. Copyright {\textcopyright} 1991, Wiley Blackwell. All rights reserved