A previously unknown group of flies is described whose males exhibit directional asymmetry, in that the left wing is larger than, and of a different shape from, the right wing. To our knowledge, wing asymmetry of this degree has not previously been reported in an animal capable of flight. Such consistent asymmetry must result from a leftÂright axis during development, a level of differentiation whose existence has been questioned for insects. Wing asymmetry of this magnitude has implications for questions in areas of development, natural selection, flight, mate selection and communication in insects. The 'handicap principle' provides a possible explanation: females will choose a mate with the greatest handicap because his survival, in spite of his handicap, is a measure of his genetic superiority.