Jasmonic acid (JA) is a key molecule initiating plant defensive responses to attack by pathogens and herbivores.  This phytohormone is produced at sites of insect damage and is ingested by feeding insects, but its occurrence in insects remains to be studied.  We report the presence of JA in eggs and neonates of all nine lepidopteran species that we screened, representing four superfamilies and five families of Lepidoptera.  Concentrations of JA in some lepidopteran species far exceeded those found in most plant species.  Levels of JA varied significantly among species and between eggs and neonates of the same species.  In some cases, eggs contained significantly more JA than neonates, but for at least one species (Lymantria dispar) neonates had significantly more JA than their eggs despite lacking food upon emergence.  The presence of JA in eggs and neonates across a wide taxonomic range may indicate that JA has an as yet undescribed function in insects.