Head capsule widths of 10,677 larvae of the gypsy moth,Lymantria disparL., from different populations in Quebec were measured and compared with those observed in Massachusetts 90 yr ago. Mean head capsule widths for each of the six instars were almost identical to those observed 90 yr earlier. Larval growth ratios decreased with molt number in all populations studied. This disagrees with Dyar's hypothesis, which proposes a geometrical head capsule growth in Lepidoptera. Populations that showed the greatest departures from Dyar's hypothesis suffered the highest levels of natural mortality. Parasitism and NPV infection are suggested as stressing factors that could have modified gypsy moth larval growth.