Abstract: Measurements were made of the acoustic target strengths of siphonophores swimming freely in the ocean. The measurements were made possible by use of a remotely operated vehicle (ROV) on which both acoustics and video camera equipment were mounted. The acoustic transducers and camera were aimed at the same volume of water and data from the two sets of devices were co-registered. The cameras were used to help search for and identify the animals and to direct the path of the ROV; so that once the animals were found they would be in or near the acoustic beams. The data show that these gas-bearing zooplankton have relatively high target strengths due to the presence of the gas inclusion. These data are essential in the use of acoustics for quantitatively surveying zooplankton for two reasons: 1) It suggests that a relatively small number of siphonophores can dominate the echo in an acoustic survey while other species present may dominate the biomass. 2) Siphonophores are very fragile and, as a result, cannot be reliably captured by use of nets. These data can help in making acoustics a quantitative survey tool for these important animals.