Variations in underwater acoustic ambient noise spectra, from 20 to 1250 cps, were analyzed using 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 seven cycles per week were observed in the spectral density functions. In several cases harmonics of the seven‐cycle‐per‐week component were also evident. The period of the cyclic variation agreed more closely with the solar day than with tidal period. The maxima occurred at approximately midnight local zone standard time, and at one location additional maxima were observed at approximately noon. The magnitudes of the periodic variation were small, 1.5 to 5.0 db, showing little evidence of seasonal dependence, except that at one location daily changes of 10 to 20 db were observed during the period of the summer solstice while hardly any periodic change was evident during the winter solstitial season. These periodic changes in the ambient noise levels were not correlated with changes in local wind speed.