A strong interference pattern has been observed in the reflection of monochromatic pings from the ocean bottom. The experiment was performed in the Gulf of Mexico at 24°16′N, 85°15′W. The acoustic pings had a frequency of 120.5 cps and a length of 0.12 sec and were repeated every sec. The transmitter was at a depth of 75 ft and the receiver at 3000 ft. The energy in successive reflections varied by as much as 6 to 1 with a mean fluctuation of 30%. This rapid change of level corresponds to that observed in the “direct” arrival which, because of the shallow source, is due to the superposition of the truly direct and the surface reflected arrivals. This is probably the principal cause of the short-term fluctuations in the bottom reflections. When these reflections are averaged in groups of ten, one obtains a moderately smooth relation between energy and range which exhibits a pronounced interference structure. This gross pattern is not due to the surface interference. The pattern observed can be interpreted in terms of a 43-ft layer having a velocity of 6280 ft/sec. (This work was supported by the Office of Naval Research.)