In Bulgaria, two different phenological forms of pine processionary moth, Thaumetopoea pityocampa (Den. & Schiff.) (Lepidoptera: Notodontidae), occur: a typical Mediterranean winter form and an early developing summer form. During the period 2017-2018, the phenological development of T. pityocampa was studied in 21 sites of pest’ range in Southern Bulgaria. The results of the observations showed that in terms of the phenology, three categories of pine processionary moth development could be distinguished: i) six sites with an early developing summer form; ii) nine sites with the typical Mediterranean winter form, and iii) six sites with larvae that overwinter both into the soil and in the nests. Besides the phenological differences in the development of both forms on the same locality, no other distinguishable morphological features were established.