To detect divergence of the introduced fall webworm, Hyphantria cunea, within its trivoltine area in Japan, developmental traits were determined in the Kobe (34°41′N) and Kumamoto (32°48′N) populations. The lower threshold temperature for development was not different between these two populations. The thermal constant for larval development was considerably smaller in the Kobe population than in the Kumamoto population. Two types of larva occurred in these populations, one with six instars and the other with seven. In the six-instar type, the developmental period was not different between the populations in males, but the pupal weight was greater and the forewing was longer in both sexes in the Kumamoto population than in the Kobe population. In the Kumamoto population, the seven-instar type had a longer developmental period, heavier pupa and longer forewing than the six-instar type. The proportion of the seven- to the six-instar type was greater in the Kumamoto population than in the Kobe population, resulting in the longer developmental time and greater body size of the former.