A strong interference pattern has been observed in the reflection of monochromatic, acoustic pings from the ocean bottom. In our experiment, performed in the Gulf of Mexico at 24°16′N, 85°15′W, the pings had a frequency of 120.5 cps, a length of 0.12 sec, and were repeated every 5 sec. The transmitter was at a depth of 75 ft and the receiver at 3000 ft. The energy in successive bottom reflections varied by as much as 6 to 1, with a mean fluctuation of 30%. This rapid change in level was largely owing to the fact that the source was shallow and the air‐water interface was rough. Averaging the bottom reflections in groups of ten produces a moderately smooth relation between signal level and range. This relation exhibits a pronounced interference structure. The pattern observed can be interpreted in terms of a 130‐ft bottom layer having a velocity of 6650 ft/sec.