Excess acoustic attenuations in the frequency range 5--160 kHz have been used to deduce densities of bubbles of radius of 15--300 μ at depths of 3--36 m in water up to 1 km deep. The measurements were made over an acoustic path 1--5 m long. Correlations have shown dependence on season, sundown and sunrise, wind, and presence or absence of sea slicks and windrows. These inferred microbubbles are evidently of biological as well as physical origin.