SUMMARYUsing a standard 9 in. diameter disc‐dropping suction trap, the aerial density ofAnisopus fenestralisabove bacteria beds was assessed hourly on 410 24‐hourly occasions over a period of 19 months, and the prevailing temperatures and wind velocities were recorded. Between o and 10 m.p.h. wind velocity there was a gradual rise in catch associated with a rise in temperature to an optimum, beyond which the catch decreased. This optimum fell as wind velocities rose; it occurred between 70° and 75° F. at winds between 2 and 4 m.p.h., at 65–70° F. with winds between 4 and 6 m.p.h. and at 60–65° F. between 6 and 8 m.p.h. At all temperatures between 30 and 80° F. the catch fell as wind velocity increased from o to 10 m.p.h. By comparing the dusk catches on successive days a significant positive correlation was found between the deviation in log catch from a 3‐day running mean and the corresponding temperature deviation, except when the wind velocity deviation exceeded + 2. There was also a significant negative correlation between deviation in log catch from the running mean and the corresponding deviation in wind velocity, except when the temperature deviation exceeded — 2.The average regressions showed that for each °F. rise in temperature the log catch was raised by 0.14 and for each 1 m.p.h. rise in wind velocity the log catch was decreased by 0.19. Expressed arithmetically the catch was doubled by a 2.2° F. rise in temperature or by a fall in wind velocity of approximately 1.5 m.p.h.The aerial density was affected not only by weather but also by fluctuations in theA. fenestralispopulation within the bed arising from direct and indirect climatic effects on its food supply—the microbial growths within the bed. Lower winter temperatures favour an accumulation of food, so the patterns of seasonal incidence of the flies from year to year are affected by the severity of