Hystological study of hungary imagos of Dermacentor pictus Herm. and other ticks of the genus Ixodes has made it possible to improve the method of physiological age determination of ixodids (Balashov, 1961). The following criteria for age determination are suggested: 1) size of sections of midgut appendages and their structural peculiarities, 2) height of midgut epithelium, 3) amount of haemoglobin inclusions in midgut cells, 4) amount of haematin in midgut cells, 5) cellular structure of midgut epithelium. Four main physiological ages can be determined by means of the above method (Razumova, 1977), which correspond to four degrees of nourishing according to Balashov (1961). By means of the method a stereotype of seasonal changes in the age structure of C. pictus population has been found out--a younger composition in spring and old one in autumn.