The diel patterns of the communal roosting behaviour of the dragonfly Potamarcha congener in the Calicut University Campus are described. The average sex-ratio observed in the dormitories, some remarks on the factors such as the nature of the roosting site, light and group effect which influence communal roosting and on the benefits that may be derived from such roosting, are given. The probable role of some of the special attributes of these dragonfly aggregations in orientation to the fixed roosting site day after day possibly by visual or pheromonal cues, or both of these, are discussed.