The interaction of plants with certain types of parasites leads to the formation of galls, organised structures that create the habitat of the parasite, caused by an abnormal proliferation of host plant's cells under the influence of growth regulators, secreted by the parasite, or by the plant itself under the influence of the parasite. Arthropods, mites in particular, are the largest group of gall-inducing phytoparasites, but the mechanisms of their interaction with plants remain virtually unexplored. The interaction of the gall-inducing eriophyoid mite Fragariocoptes setiger with Fragaria viridis plants was used as a model gall–mite system where data were obtained on the changes in the histological structure of F. viridis leaf blades under the influence of the mites as well as F. viridis gene expression during gall formation. For histological purposes, gall formation was split into four stages with each corresponding to the age of the gall as well as to specific changes that occur during that period. A dramatic change of adaxial–abaxial polarity of the lamina throughout the four stages was observed. Moreover, qRT-PCR analysis of F. viridis gene expression in the developing gall revealed changes in the expression levels of certain meristem-specific genes, as well as the genes that determine adaxial–abaxial polarity and signalling of phytohormones.