Field experiments using stems ofSpartina allernifoliaas substrata indicate thatChthamalus fragilis, one of the few chthamalids to be so tested, shares with all the balanids so far examined, the advantage of settling more readily among conspecifics than on bare substrata. Despite a general preference for concavities, C.fragiliscyprids settle on the convex stems ofSpartina, particularly if they already bear settled individuals. Initial settlement usually occurs in the axillary region, the prime location, which is shaded by the blade. Later colonisers tend to settle further up the stem as space in the axillary area becomes filled up. As in other barnacles, all individuals appear to settle so that the carina points upwards and the adult eyes point to the shaded side.