Technical Abstract: Melaleuca quinquenervia (melaleuca, paperbark tree) is an exotic invasive tree in Florida, Hawaii, and some Caribbean islands. Puccinia psidii (guava rust-fungus) is a Neotropical rust fungus, reported to attack many species in the Myrtaceae and one genus in the Heteropyxidaceae, both members of the Myrtales. This rust attacks succulent stems and foliage of melaleuca, causing twig dieback and defoliation. Throughout its adventive range in Florida melaleuca plants, growing adjacent to each other and under similar growing conditions, were observed to exhibit either susceptible or resistant reactions towards this fungus. To quantify the percentage of susceptible and resistant plants, we visually evaluated 331 melaleuca trees for rust-fungus symptoms. The percentages of susceptible, resistant and immune plants were 85.8, 13.0, and 1.2%, respectively. Additional screenhouse studies corroborated these trends observed in the field. Susceptible, resistant and immune seedlings obtained from unknown seed sources constituted 63.3, 33.6, and 3.2%, respectively. Two experiments conducted 6-yr apart using clonal materials from resistant and susceptible lines confirmed the persistence of resistance or susceptibility. These results show that the current strain of this fungus is restricted to a portion of melaleuca populations though it is capable of infecting plants in other Myrtaceous genera in Florida and the selection of resistant genotypes/biotypes of susceptible host species in other plant systems of economic and environmental importance may be possible by identifying naturally-occurring resistant individuals in affected plant populations. Additional studies will be needed to ascertain the attributes of this rust-fungus/melaleuca association that confer virulence or resistance.