How do males of the amphipod crustacea distinguish the females ? It is well known that the males of the Gammaridea have the curious habit of carrying the females under their body for a considerable time. This act of transportation has probably no further significance in relation to the fertilization of the eggs than to secure the proximity of the two sexes when the proper time for fertilization arrives. According to the observations of Della Valle on Gammarus pungens the eggs are not fertilized until after they are laid, oviposition occurring a short time after moulting. When the moulting of the female has been effected, the male bends his body beneath that of his mate and deposits spermatozoa upon the ventral surface of her thorax. The deposit of sperm is followed within half an hour by the laying of the eggs. After the act of copulation the male regains his original position and swims about with the female as before. The same relation of oviposition to moulting was found by Miss Langenbeck in Microdeutopus, the male leaving the female during her moulting process but soon resuming his previous position when the moult was completed. The instinct of the male amphipod to seize and retain hold of the female is one of remarkable strength. The male retains his hold, despite all efforts to dislodge him, with remarkable persistence, and will still cling to the female after the posterior half of his body has been cut away. My own observations on the sexual behavior of amphipods relate mainly to three species, Amphithoe longimana Smith, Hyalella dentata Smith and Gammarus fasciatus Say. The sexual behavior of these three species is remarkably similar, athough they belong to as many distinct families. The female while being carried about keeps remarkably impassive. Her thoracic legs are drawn up, the abdomen held strongly flexed, the whole body assuming as compact a form as possible. She takes little or no part in swimming; the movement of the