The hemlock looper, Lambdina fiscellaria, is one of the most economically damaging defoliators of North American coniferous forests. Basic information on its reproductive biology is an essential prerequisite for understanding its population dynamics. Realized fecundity for the two major hemlock looper ecotypes varies along a latitudinal gradient in eastern Canada, but their daily oviposition patterns are similar. Mated females lay eggs after a short pre-oviposition period, with daily oviposition peaking in the first 3 days. Mated females lay significantly more eggs than unmated ones, the latter laying their eggs more evenly throughout their life. Eggs deposited early in the oviposition period are larger than those deposited near the end. This may influence over-winter survival of hemlock loopers and should be considered in studies to better understand the population dynamics and improve management of this defoliator.