It is shown analytically that even for waves whose amplitude is small compared with boundary-layer thickness there are at least three possible solutions for the mass-transport velocity in the interior of the fluid. Masstransport velocity profiles were observed in a wave tank and were found to be similar to those given by one of these solutions but significantly different from LONGUIT-HIGGINS' "conduction" solution even for waves of very small amplitude. It was also observed that when the wave amplitude dropped below a certain value the mass-transport velocity in the boundary layer at the bed ceased to be in the direction of wave propagation.