Many authors noted the attraction of the males of several species of mosquitoes to sound (references summarized by Roth, 1948). The frequencies of sound to which the males respond were investigated by Roth (1948) and the frequencies and intensities by Wishart and Riordan (1959). Tischner (1953) applied electrophysiological methods to the reception of sound in the male of Anopheles subpictus Grassi and advanced a theory to explain the ability of the male of this species to orient itself toward the sound source. In his theory he dismissed the use of phase differences between the two receptors because of the relatively small distance between the two antennae and concluded that each is capable of functioning alone by detecting the relationship bemeen the fundamental and the second harmonic in the microplionics produced by the Johnston's organ. He did not demonstrate this by behaviour experiments nor, do we believe, did he produce sufficient electrophysiological evidence. The present paper is a further examination of this subject in the mosquito Aedes aegypti (L.), in an attempt to reconcile the data from electrophysiological experiments with those of behaviour in an explanation of the orientation of male mosquitoes towards sound.