Data from flight traps and 5-minute counts of weevils observed in flight indicated that most alfalfa weevils, Hypera postica (Gyllenhal), flew between 5 and 11 PM (especially 6:30–8:30 PM), very few between 11 PM and 9 AM, and some between 9 AM and 5 PM. At any given moment, the accuracy of the flight-trap method as an estimator of the density of flight was highly dependent on wind speed, though over a period of several hours it gave about the same results as the flight-count method. Pre- and post- take-off behavior of the weevil was like that of other migrating insects. Adults ascended to plant tips, the site of feeding as well as take-off, earlier on evenings when large numbers flew and later on evenings when there was little or no flight. The factor most strongly influencing flight activity was light intensity. A decrease in light, rather than a certain low threshold value of light, seemed to initiate flight, but only after the condition of sufficiently high temperature had been satisfied. In most fields, substantial afternoon flight occurred only if lower light intensity was accompanied by sufficiently high relative humidity. However, sudden removal of the alfalfa, as in harvest, created conditions so unfavorable to the weevil that (soil temperatures permitting) large numbers flew, irrespective of light intensity.