The properties of acoustic interneurons in fiddler (Uca pugilator and U. minax) and ghost (Ocypode quadrata) crabs are described, as revealed by tests with pure tones. Three types of interneurons were present in all species: tonic, which fired for the duration of the stimulus; phasic, which fired briefly at the onset of stimulation; and inhibited, in which ongoing spontaneous activity was suppressed during stimulation. Spectral sensitivities of the cells differed among the species and were "matched" to the energy composition of the sounds. The combined properties of the cells were suited to resolve the most likely temporal differences between the sounds of each species: pulse repetition rate, sound duration, and interval between successive sounds. None of the cells coded tone frequency. The responses of all cells were suppressed during locomotion (walking) by the preparations. Finally, all interneurons were "nonhabituating" and in this respect resembled their functional analogues in cricket and grasshoppe...