Five ultrasonic devices generating fundamental frequencies of 20-70 kHz were tested for their efficacy in repelling mosquitoes. Four species (Anopheles quadrimaculatus, Aedes aegypti, Ae. triseriatus and Haemagogus equinus), were used in a flight chamber in which females must fly upwind against the direction of the sound waves and around the ultrasonic devices to reach a trap downwind of a source of human breath and skin emanations. Repellency was rated by the number of mosquitoes entering the trap during a series of 5 min tests. For all species there was no significant difference between the numbers trapped when the devices were switched on or off, when all devices were tested simultaneously. Tests of individual devices against Ae. aegypti also failed to show a repellent effect.