Time-averaged sediment layer sound speeds, or interval velocities are generally obtained from source-receiver separations at various offsets. In marine environments, the traditional measurement technique employs a towed broadband source and a long towed receive array, often kilometers in length. A method is described for estimating interval velocities using a simpler, monostatic configuration. The method is first tested using simulated data from various layered seabed structures with roughness at each interface; the resulting estimated interval velocity is within less than 1% of the true value. Monostatic measured data from the Gulf of Lion are also presented which exhibit many characteristics similar to the simulated data. The method applied to the measured data yield an interval velocity of 1569 m/s in an 18 m sediment layer. This accords with nearby independent data from cores and wide-angle reflection analysis. [Work supported by ONR Littoral Geosciences and Optics program.]