Because of the misuse of the term ’phoresy’, it is redefined as the attachment of one animal to another only when the attached animal actively sought a host, neither fed nor changed ontogenetically during attachment, and attached temporarily. Phoretic behaviour was investigated in the mite Macrocheles muscaedomesticae (Scop.), which is phoretic on the adults, and predacious on the eggs, of Musca domestica (L.). Phoretic attachment increased, and detachment decreased, with an increase in age of the manure substrate. The principal effect of age on manure apparently was the gradual decrease in moisture content. The addition of water to dried manure restored its attractiveness. The attractant component of manure is evidently not an ammonia-containing compound, judging by the general lack attraction of the mites to a series of such inorganic compounds. Since immobilised flies attracted many more phoretic mites than freshly killed flies, it is probable that the attractant in the flies is not simply movement. It was concluded that phoretic attachment occurs when the attractiveness of the manure exceeds that of the flies. This conclusion is in general agreement with the only other extensive investigation of phoretic behaviour in mites.