It is commonly believed that gall inducers have the ability to control and program the host plant growth to their own benefit. The pattern of changes in the contents of reducing sugars, protein and phenolic compounds as well as the activity of chitinase and β-1,3-glucanase in galls, galled and intact leaves of Ulmus pumila were investigated during three stages of Tetraneura ulmi gall development. High protein accumulation in galls at the initial period of gall formation, increased biosynthesis of total phenolics during galling process, and the highest activity of pathogenesis-related protein at the stage of mature galls were detected. Therefore, it can be suggested that T. ulmi can manipulate the biochemical machinery of the galls for its own needs.