Areas of sand dunes and ripples have been observed in the estuary of the St. Lawrence River below Quebec City. A train of dunes 18 km long is present in the channel of Ile-aux-Coudres. Symmetrical dunes as high as 12 m and reaching 120 m in wave-length are abundant. Symmetrical subsurface crests are also revealed by seismic reflection profiling. The mode of origin of these dunes is not yet established. However, because of the high degree of stratification present in the water column, the possible role played by internal water waves in their formation should be considered.