Four cylindrical mosquito bait traps were modified by adding a vertical partition of screening to keep separate the mosquitoes that entered each end. When these traps were tested with the horizontal axis in 4 orientations (parallel to wind direction, perpendicular to wind direction, at a randomly chosen angle, and continuously turning about a vertical axis), the revolving trap caught the most Culex nigripalpus and the most Psorophora confinnis, the wind direction trap caught the fewest C. nigripalpus, and the crosswind trap the fewest P. Confinnis. Total catches of C. nigripalpus, though dependent on the basic orientation of a trap, were apparently unrelated to the proportions caught in the 2 ends. The success of the different orientations was related to the velocity and directional range of the wind. The superiority of the revolving trap is explained by postulating an optimal angle of orientation, dependent on wind velocity, through which the trap turned 4 times in each complete revolution.