Vertical orientation of shells of the common American oyster Crassostrea virginica is widespread on three large oyster reefs located off the entrance to the Suwannee River on the Gulf coast of Florida. The orientation is found on the surfaces of the reefs, particularly on and just west of actively building shell bars that occur along the eastern margins of the reefs. The orientation consists largely of densely packed right valves and right valve fragments of Crassostrea virginica and tends to form shell patches of less than two square meters. Mean sizes of the vertically oriented shells ranged from approximately -1.80 phi to -4.20 phi . Long axes of the shells tend to be aligned horizontally rather than vertically. A majority of the shells lie in positions that are either transverse or diagonal to the prevailing current directions. The vertical orientation appears to represent a position of stability for the oyster shells in a regime dominated by tidal currents.