Fifteen silver birch (Betula pendula Roth) lines carrying a sugar beet chitinase IV gene and eight wild-type birch clones were grown in a field trial. The composition and density of the insect population and the leaf damage caused by insects were monitored and compared between transgenic and wild-type trees. The most abundant insect group in all trees was aphids, and the variation in total insect densities was mainly explained by the variation in aphid densities. Insect densities were generally higher in the transgenic than in the control trees, indicating that the expression of the sugar beet chitinase IV gene had an influence on the suitability of birch leaves to aphids. The level of leaf damage was higher among transgenic than among control trees. Chewing damage was the most common type of leaf damage in all trees. The number of different damage types was higher among the wild-type clones than among the transgenic lines or their controls. The results indicate that the chitinase transgenic trees are more susceptible to aphids and suffer higher levels of leaf damage than the control trees. In the composition of the damage types, the control trees were more similar to the transgenic than to other wild-type trees, indicating that the composition was mostly linked to the genotype of the tree and not to the expression of the transgene. This study provides important information on the ecological interactions of chitinase transgenic trees in the field trial. No clear harmful effects of transgenic chitinase on the biodiversity of insect population were detected.