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A sliding mode control law is presented and experimentally implemented for trajectory tracking of underactuated autonomous surface vessels. The control law is developed by introducing a first order sliding surface in terms of surge tracking errors and a second order one in terms of lateral motion tracking errors. The resulting sliding mode control law guarantees position tracking while the rotational motion remains bounded. The vessel is a small boat with two propellers in a small indoor pool. The position and orientation of the boat is measured using a camera and with two infrared diodes attached near the front and back ends of the boat. A computer with controller board processes the camera image, calculates the control forces and their corresponding input voltages, and sends the control signals to wireless receivers on the vessel using a wireless transmitter. Several experiments are performed where the vessel follows straight-line trajectories fairly accurately.