The arm test for mosquito repellency (King, 1954) is a useful criterion for selecting compounds that are both persistent and repellent and therefore potentially useful in the field. However, if repellency is to be correlated with other molecular properties, a criterion must be found by which intrinsic repellency can be recognized independently of properties like volatility or solubility in sweat (Wright, 1958). Since the prime requirement of a good mosquito repellent is that it shall prevent alightment, some motion pictures were taken of mosquitoes in the act of alighting or failing to alight on variously treated surfaces. The photographs revealed an alteration in the flight posture which was then investigated in detail.