No explicit higher-level cladistic analysis exists for Lasiocampidae, the largest family of Bombycoidea (s. lat.). As a start toward such a phylogeny, we sequenced the protein-encoding, nuclear gene elongation factor-I α (EF-1α) from 13 lasiocampids representing all the subfamilies and tribes present in North America, plus three outgroups. Phylogenetic signal, mainly from synonymous change, was strong, with seven of 11 potential groups in Lasiocampidae showing > 90% bootstrap support under parsimony and likelihood analyses. Separation between the two American subfamilies, Lasiocampinae and Macromphaliinae, was strongly supported. Within the largest tribe, Lasiocampini, EF-1α strongly resolves a clade of unusually large lasiocampids, containing a New World element ((Eutachyptera, Gloveria), (Prorifrons, Quadrina)) with a Eurasian sister group of Macrothylacia + Dendrolimus. Resolution of deeper relationships within Lasiocampinae, however, including those among tribes, is presently weak and will probably require expanded taxon sampling. The 'large Lasiocampini' group shares a possibly synapomorphic life cycle (not expressed in its tropical members) that includes overwintering by larvae. Larval sociality is found in three of the 'large Lasiocampini' genera, each of which may represent an origin separate from the others and from Malacosoma.