A study is made to determine the frequency and concentration dependence of the ultrasonic attenuation in aqueous suspensions of hatched and unhatched brine shrimp in the frequency range 1–45 Mc. In both types of suspensions a linear relationship is found to exist between the excess attenuation and concentration at each frequency. The attenuation caused by hatched brine shrimp is found to be the same when the organisms are either living or dead.For both types of suspensions the major cause of the excess attenuation is attributed to a scattering mechanism. Over the range of frequencies studied, while the excess attenuation in a hatched brine shrimp suspension varies as the 1.35 power of frequency, the attenuation caused by unhatched brine shrimp exhibits a maximum. The fact that the unhatched brine shrimp can be considered as small spherical particles allows calculation of the scattering loss, which, when subtracted from the measured values, results in a resonance type loss. The measured loss maximum associated with this resonance occurs at a frequency coincident with the principle mode of oscillation calculated assuming the eggs to be spherical cavity oscillators. A simple theory of geometric scattering from unhatched brine shrimp gives attenuation values which agree exactly with those measured above 20 Mc.