The difference in acoustic impedance between air and water makes it difficult to communicate efficiently with sound in both media. Sounds made in either air or water, for example, are almost completely reflected at the boundary. Several species, however, may use amphibious communication. Here I report that hippos make sounds both in air and underwater, and, with their heads in an amphibious position (eyes and nostrils above water but mouth and throat submerged), are able to transmit sounds to both media simultaneously. Hippos on the surface respond to both natural production and playback of the surface component of a simultaneous amphibious sound (SAS) by calling in the amphibious position (territorial alpha males usually call first). Hippos underwater consistently surface immediately following an SAS, or the playback of the underwater component of an SAS, and usually call in a chorus that can spread in air from one territory to the next for long distances. I also found that hippos in the amphibious position respond to playback of underwater hippo screams that are inaudible in air, and also to natural underwater calls, suggesting that they can hear in air and water simultaneously. I discuss behaviour associated with amphibious communication and probable mechanisms for producing and receiving these sounds.