Underwater sound transmission has been studied as a function of range at frequencies of 0.2, 0.6, 1.8, 7.5, and 22.5 kc, under a variety of thermal conditions. At the three lower frequencies, marked interference patterns are observed. These result from the combination of surface reflected sound (as from an image source) and directly transmitted sound. Refraction modifies the interference pattern. When the sound velocity increases with increasing depth, the interference pattern is contracted; when the velocity increases with depth, the pattern is extended. An easily used formula is developed to predict this influence of refraction on the propagation of sound emitted from a point source adjacent to a reflecting plane. The theory is in good agreement with the experimental data obtained with the source at a depth of 14 feet from the surface and the receiver as deep as 300 feet.