Although Cameraria ohridella is a pest mining Aesculus hippocastanum leaves, has been known in Europe for 30 years, there is still no completely efficient method for control of this insect. The defence strategies of A. hippocastanum leaves against infestations by this pest have not been fully elucidated yet. The aim of the present study was to identify the defence strategy adopted by A. hippocastanum against the pest based on anatomical and histochemical analyses of leaves as well as to provide details of the morphology and behaviour of the developmental forms of C. ohridella. The study was carried out using a scanning, light, and fluorescence microscope. It was demonstrated that the response of the A. hippocastanum to C. ohridella larval infestations consisted in enhanced accumulation of phenolic compounds in tissues located at the border of mines and in the thickened layer of the leaf supporting the cocoon. Thorough analyses of the consecutive developmental stages of C. ohridella revealed new previously unknown features of larva L1 and some details of the mandible structures of feeding larvae L1-L4 and the characteristics of the cocoon structure, which is produced by larvae L5 and L6. We have found six stages in the developmental cycle of this pest in the present study.