The temporal and spatial variations of the index of refraction, μ, cause fluctuations of sound phase and amplitude that can be understood only by defining μ in terms of the duration, location, and range of the acoustical experiment. A “universal” isotropic spatial correlation function of μ has been derived from a simplified form of the Kolmogorov‐Batchelor spectrum of temperature fluctuations in a turbulent medium. However, such a correlation function, although more accurate than the popular Gaussian form, appears to be correct only for spatial lags less than 1 or 2 m at sea. Furthermore,in‐situexperiments have confirmed that the common presence of ambient bubbles in the upper several meters of the ocean produces a microstruc‐ture of the index in which the temperature is much less important than the bubble distribution. In this bubble‐dominated region the average speed of sound, the variance, and the frequency spectrum of the fluctuations of the speed are all strong functions of the sound frequency, the ocean wave spectrum, and the depth of the experiment. The phase fluctuations due to propagation in the upper ocean volume are compared with those occuring during forward scatter from the surface itself. [Research support by Naval Ship Systems Command.]