Participation in man-in-the-sea experiments Tektite I and II permitted an examination of the orientation of pelecypod shells and shell fragments on sediment unaffected by waves or currents. Whether on vegetated or unvegetated unrippled sand or on solid coral, the particles lie predominantly with concave sides up. This orientation occurs both for open articulated empty valves and for isolated disarticulated valves. The degrees of orientation differs with shell size; the smaller shells are less preferentially oriented concave-up than are the larger shells. The concave-up orientation results from the activity of predators and scavengers, from bioturbation, or from a combination of the two mechanisms. An experiment that exposed nearly 1200 shells to 40 days of bioturbation demonstrated that disturbance by organisms is an effective agent for producing a dominantly concave-up orientation. Shell size and shell shape are significant factors in bioturbational rotation. In general, the tendency to be rotated to a concave-up position increases with increasing shell size to the point where shells are too large to be readily overturned by most organisms. The percent of concave-up shells also depends on the angle of balance of the shell; shells rotated by bioturbation tend to behave like loaded dice. Shells that became buried during the experiment showed a more random orientation than did those exposed on the surface after 40 days. The orientation of shells in quiet water differs markedly from those orientations produced under most conditions of waves or currents. Although concave-up assemblages can be produced by deposition from turbidity currents or from the transport of shells across small ripples, analysis of orientation as it relates to shell size and shape may provide a means of identifying specific depositional mechanisms or conditions.