INTRODUCTION A controversial point in the understanding of the feeding mechanism of viperid snakes is the use of the poison fang in the swallowing mechanism. Some authors believe that the poison fang takes an active part in the swallowing act, although the majority supports the view that the fang is, at least partly, held down (Klauber, 1956). Which kind of construction in the jaws is responsible for the difference between the actions of the fang in striking and swallowing again is a matter of dispute. The pterygoid-ectopterygoid joint, which by most authors (Kathariner, 1900; Willson, 1908; Haas, 1931; Klauber, 1939, 1956) is considered stiff or immovable, may play a crucial role. There is also some disagreement on the strike being either a stab (Van Riper, 1953) or a true bite. This paper reports on new information to solve these problems. The mechanism itself is interesting from a general functional-anatomical point of view, since it presents a case of formation of a delicate structural balance to meet two contradictory demands in a part of the feedingconstruction. The presence of a large poison fang apparently has far-reaching consequences for the construction of the swallowing mechanism. MATERIAL AND METHODS The main observations were made with Crotalus ruber ruber Cope. Additional information was obtained from some Crotalus viridis hellen Meek. The movements of the various bony elements were studied with the aid of 16 mm movies, film speed 25 frames/sec. The films were analysed with an analector, Oude Delft.