About 380 described species of Psylloidea occur on Eucalyptus in Australia. These show differences in diversification,feeding behavior, and apparent patterns of development. We analyzed the quality of nutrients used by three species of Aphalaridae belonging to different feeding guilds on three species of Eucalyptus. We evaluated the quantity and quality of total nonstructural carbohydrates (TNC), d13C and d15N isotope ratios, amino acids, and fatty acids. In general, TNC levels were greater in infested leaves than in uninfested leaves of the three species of Eucalyptus. TNC levels in the leaves of E. macrorhyncha F. Mueller ex Bentham were the highest and in E. globulus Labillardie' re the lowest. Total masses of amino acids and fatty acids were the greatest in the leaves of E. globulus infested by C. eucalypti (Maskell), followed by leaves of E. sideroxylon A. Cunningham ex. Woolls infested by the species of Glycaspis Taylor, and the lowest values were in the leaves of E. macrorhyncha infested by species of Synglycaspis Moore. In general, d13C increased in C. eucalypti-infested leaves of E. globulus,Glycaspis sp. infested leaves of E. sideroxylon, and Synglycaspis sp. infested leaves of E. macrorhyncha. Nitrogen-isotope ratios (d15N values) were not significantly different in infested and uninfested leaves. The freeliving C. eucalypti stress E. globulus leaves more intensely by its group-feeding behavior, whereas the gall-inducing species of Synglycaspis stresses E. macrorhyncha leaves in such a way to elicit a response with a novel phenotypic expression, viz., the gall. The lerp-forming species of Glycaspis utilize nutrients, especially sugars,the excess of which is secreted to build their characteristic lerp.