The swallow bug, Oeciacus vicarius Horvath, is of common occurrence in North America, chiefly, if not entirely, upon the cliff swallows, Petrochelidon lunifrons (Say); it has been recorded from the barn swallow, probably in error.It appears to be really distinct from the European swallow bug, Oeciacus hirundinis (Linnacus).The habits, structure, and specific characters of the bug do not seem to have been adequately studied hitherto.This paper deals with the external structure of the bug and of such of its biology as could be worked out under the handicap that its natural host could not be kept in captivity.The bugs attack man readily, but do not appear to adapt themselves readily to living upon man or in his dwellings.