First instars of the eucalyptus longhorned borer,Phoracantha semipunctata(F.), were not capable of colonizing bark of vigorous standing trees of twoEucalyptusspecies. The lack of a kino gum reaction after the introduction of larvae into the bark ofE. grandisHill ex Maiden andE. tereticornisSmall strongly indicates that this gum does not play an important role in the initial defense against borer attack. Larvae were also not able to colonize the bark of logs that were maintained at high moisture content but were able to colonize the bark of dry logs and artificially water-stressed trees that had reduced bark moisture content. We propose that bark moisture content plays a critical role in the resistance of eucalyptus trees against colonization by eucalyptus longhorned borer larvae.