Surfaces on which the cypris larvae of barnacles do not settle readily were treated with aqueous extracts of whole barnacles. Cyprids then settled on the treated surfaces as readily as they do on surfaces bearing adult barnacles, thus simulating the gregarious response to their own species. It is inferred that a settling factor, present in barnacle tissues, was transferred to a neutral surface rendering it favourable for settlement. The settling factor was shown to be non-dialyzable and stable to boiling in aqueous solution. When attached to a surface, it resisted severe physical and chemical treatments. The settling factor evoked a greater response from cyprids of the species used in preparing the extract than from cyprids of other species. Extracts made from other cirripedes and from all other arthropods tested, promoted settlement, but were less effective than extracts made from the same species of cirripede. The settling factor was found to be located mainly in the integument of arthropods and was present in greater amount in newly moulted individuals. It is suggested that the settling factor to which the cyprid responds may be the arthropodin of its own species, and that arthropodins of related species evoke a similar response. No evidence was found for chemotaxis, the cyprids responding to the setting factor only after alighting on the treated surface. It is thought likely that cyprids, in common with the larvae of many other sedentary marine organisms, may not be capable of detecting or responding to such chemical releasers in the sea unless they are in contact with the surface on which the chemical is adsorbed or from which it is diffusing.