Eastern hemlock (Tsuga canadensis (L.) Carriere) is allelopathic to its own regeneration. Aqueous extracts of hemlock litter caused a 74 percent reduction in seed germination and 100 percent mortality in 6-day-old seedlings of eastern hemlock. Root, mycorrhizal root, and litter extracts, however, had no effect on hemlock seedlings older than 2 weeks. Red oak (Quercus rubra L.) showed a 52 percent reduction and grasses a 10 percent reduction in seed germination due to litter extract treatment. Oak seedlings were not inhibited by litter extracts. Grasses showed significantly greater inhibition of seed germination due to root and mycorrhizal root extracts than to litter extracts. Lettuce (Lactuca sativa L.) seed germination bioassays placed seven hemlock extracts in the following order of inhibition: Root bark > root, mycorrhizal root » litter > stem bark, foliage, soil. Lettuce root growth bioassays placed seven hemlock extracts in the following order of inhibition: Root bark > stem bark, root, mycorrhizal root » litter > foliage, soil. Forest Sci. 28:681-686.