Gypsy moth instars four through six move down trees at dawn and climb back up at dusk. This daily migration appears to be influenced by environmental conditions, including abiotic factors. In this study, gypsy moth caterpillars were given choices of abiotic conditions to see what their preferences are. Different gypsy moth instars responded similarly in a temperature gradient, were indifferent to direct infrared radiation, and moved upward in a uniformly illuminated tube. All instars tended to accumulate at the lower of two relative humidities when given a choice. Small larvae, but not large ones, were strongly attracted to light. Under a laboratory photoperiod of 16 h light per day, fifth instars migrated away from a food source during the lighted period and toward the food when it was dark. I conclude that sunlight is probably responsible for stimulating large caterpillars to migrate up and down trees.