The purpose of this study was to investigate variation in selected anatomical, physical and mechanical properties of three Populus x euramericana (Dode) Guinier hybrid clones grown on two dissimilar sites in Hungary. Six 15 years old trees from three clones on one site and six 10 years old trees from two clones on the other site were sampled at breast height. Anatomical properties, including vessel lumen diameter, area and shape factor, number of vessels per unit area, fiber lumen diameter and area, fiber length, ray area, and cell wall area percent were measured by an image analyzer. Site, clone and/or their interaction significantly affected one or more of these properties except fiber length. Variations were significant among trees within clone and site for all variables except vessel lumen diameter. However, most of the variations was within tree as a result of the effect of age. Statistically significant correlations were found between anatomical properties. For specific gravity, there was no significant differences between sites but there was a significant clone effect with a repeatability of 0.51 indicating genetic control on this property. There were no significant differences among clones for modulus of rupture, crushing strength, maximum tensile strength, and tension modulus of elasticity. Strength properties were significantly higher near the bark than close to the pith except for maximum tensile strength. Specific gravity was not the single most important factor affecting strength properties of the clones. The effect of age and growth rate on specific gravity and anatomical properties were also investigated. Differences among clones for growth ring widths were significant in the "good" years only. Specific gravity was high near the pith, then each of the clones exhibited a different radial pattern. Based on segmented regression, a quadratic model with a plateau proved to be useful for estimating the demarcation between juvenile and mature anatomical characteristics. The ages of maturation were not the same for all properties; however, the order of maturation was the same on both sites. No consistent relationships between growth rate and specific gravity and anatomical properties were found when growth rings of the same age were compared. / Ph. D.