The free amino acid composition and concentration in young and mature leaves of Eucalyptus blakelyi were determined. These data are compared with published data on the total nitrogen in similar tissue.Variation in total nitrogen of the host plant, induced by leaf age or environmental conditions, has been reported to influence phytophagous insect population dynamics [1, 2]. The performance of two species of aphids has been associated with host plant age and changes in soluble N and individual amino acids [3]. Sap-sucking psyllids (Insecta: Homoptera), of the genus Glycaspis favour young foliage [4,5], In order to investigate further the hypothesis that this preference is a function of variation in free amino acid composition and concentration with leaf age, it is desirable to quantify the ammo acids in leaf tissue of the host tree species of Glycaspis, Eucdyptus blakelyi Maiden. In this communication are presented the results of an investigation into the free amino acid content of young and mature leaves of the host.