The density and sound speed of two coastal, gelatinous zooplankton, Mnemiopsis leidyi ͑a ctenophore͒ and Cyanea capillata ͑lion's mane jellyfish͒, were measured. These parameters are important inputs to acoustic scattering models. Two different methods were used to measure the density of individual animals: one used a balance and graduated cylinder to determine the mass and displacement volume of the animal, the other varied the density of the solution the animal was immersed in. When the same

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... nimal was measured using both methods, density values were within 1% of each other. A travel-time difference method was used to measure the sound speed within the animals. The densities of both zooplankton slightly decreased as the animals increased in length, mass, and volume. The ratio of animal density and sound speed to the surrounding seawater ͑g and h, respectively͒ are reported for both animals. For Mnemiopsis leidyi ranging in length from 1 to 5 cm, the mean value ͑±standard deviation͒ of g and h were 1.009 ͑±0.004͒ and 1.007 ͑±0.001͒. For Cyanea capillata ranging in bell diameter from 2 to 11 cm, the mean value ͑±standard deviation͒ of g and single value of h were 1.009 ͑±0.004͒ and 1.0004.