Adult female horse flies ( Tabanus spp. L.) were collected hourly in modified Malaise traps. Regression analyses of numbers of flies collected and the values of various meteorological factors in the ranges observed indicated which factors influenced activity. From greater to lesser influence, these factors were: barometric pressure, temperature, evaporation and evaporation change per hour, total sky radiation change per hour, total sky radiation and wind velocity change per hour, and relative humidity and temperature change per hour. The influence of these factors upon activity of the 6 most abundant species was also determined. Prediction equations are presented and the traps described.