Technical Abstract: Cactoblastis cactorum (Lepidoptera: Pyralidae) spends most of its larval life feeding within the cladodes of Opuntia cactuses, but the gregarious caterpillars begin their life outside the plant, and in the later instars make intermittent excursions over plant surfaces to access new cladodes and to thermoregulate. The study reported here shows that when the caterpillars move outside of the plant, they mark and follow trails that serve to keep the cohort together. Artificial trails prepared from hexane extracts of the caterpillar's paired mandibular glands were readily followed by the caterpillars. The glands are remarkable large and their fluid contents, which constitute approximately 1% of the total wet mass of a caterpillar, are secreted onto the substrate as they move about. Although the caterpillars also lay down copious quantities of silk, the material in itself neither elicits trail following nor is it a requisite component of pathways that elicit trial following. Previous analyses of the mandibular glands of other species of pyralid caterpillars show that the oily nature of their contents is attributable to a series of structurally distinct 2-acyl-1,3 cyclohexane diones which constitute the major components of the glands. Chemical analysis show that the glands of C. cactorum contain structurally similar compounds and bioassays indicate that trail following occurs in response to these chemicals. While the mandibular glands fluids have been shown to act as semiochemicals, effecting both interspecific and intraspecific behavior in other species of pyralids, the present study is the first to report their use as a trail pheromone.