Larvae of P. obliquifasciella, a consistent insect pest of Amaltas (Cassia fistula), were collected from field and their head capsule widths were measured under a binocular microscope (50X). Five larval instars were determined with maximum and minimum ratio of increase in head capsule width of 0.748 and 0.624 with an average 0.70 which conforms with Dyar's rule. The pest was found active in all larval instars from April to October except June and it over wintered in larval stage (2-5th instar) from November to February. Six to seven generations were found in a year.