Variations in underwater ambient noise spectra, from 20 to 1000 cps, were studied by use of a statistical filter technique to examine the spectral density functions in the range from 1 to 80 cycles per week. At frequencies below 100 cps narrow band components at 7 and 14 cycles per week were observed in the spectral density function. The cyclic variations agree more closely with the rotational period of the earth than with tidal cycles. The maxima occur at approximately midnight and noon, local time. These periodic changes in the ambient noise levels were not correlated with changes in local wind speed.