The number of larval instars of Prionoxystus robiniae (Peck) varied even within broods reared under the same conditions, but it appeared to be influenced by sex and possibly by heredity and temperature. Instar numbers ranged from 8 to 31 and mean head-capsule widths from 0.96 to 6.65 mm. Head-capsule widths of males and females were about equal during the early instars, but generally they became increasingly larger for females. Variability notwithstanding, the early instars could be satisfactorily separated by head widths. Growth did not fit Dyar's rule (1890 Psyche, vol. 5) of geometrical progression, but rather it increased at a decreasing rate through Instar 7 and then leveled off near a ratio of 1.00.