Aggregate formation of the 1st instar larvae of Eurydema rugosa was induced by both olfactory and tactile responses rather than visual perception. The chemical stimuli responsible for the aggregation were derived from scent components produced by the stink bugs themselves. These chemical substances were considered as "aggregation pheromone." trans-2-Hexenal, one of the major components of the scent of E. rugosa, had roles of "attractant" and "arrestant". The same aldehyde was reported to have an alarm effect for aggregating larvae of R. rugosa (ISHIWATARI, 1974), therefore, trans-2-hexenal was considered to be not only the alarm pheromone component but also the aggregation pheromone component.