This paper discusses the advantages of studying fluctuations of underwater sound in vertical transmission and points out the drawbacks of the horizontal transmission studies. Certain experiments eliminating avoidable known causes of fluctuations have been carried out at the Indian Naval Physical Laboratory and are described. The results show that the variation coefficient of fluctuation has a higher value than those reported by other workers in other areas. The suggestion put forward by Nanda that scattering by zooplankton may be responsible for this large fluctuation is supported.