Delineation of larval instars of an insect is important in morphological and physiological studies. We applied Dyar’s rule to morphometric measurements of larval instars from field populations of the yellow stem borer (YSB), Scirpophaga incertulas (Walk.) collected on two paddy cultivars over three seasons. The conventional head capsule width (HCW) measurement was compared with mandibular width (MW) of larvae from both field and laboratory populations. Frequency distribution of HCW showed seven peaks with size overlaps indicating seven instars in field populations. Distinct size classes were observed in MW with means of 0.042, 0.083, 0.125, 0.166, 0.208, 0.250 and 0.291 mm for 1^st, 2^nd, 3^rd, 4^th, 5^th, 6^th and 7^th instars, respectively, in the field population. However, size classes fell into five instars from HCW and MW measurements of larvae reared on cut paddy stems under constant temperature (25 ± 1°C) and humidity (60± 5%) indicating two additional, late larval instars under variable field conditions. For the same instar, mean HCW size slightly varied and was overlapping while mean MW size was identical between field and laboratory populations for each of the first five common larval instars.