Apparatus was designed for testing the frequency of flights of aphids under different conditions of temperature, relative humidity, light and pressure. Young aphids (1–4 days after metamorphosis) flew more often and showed less individual variability than older ones; with all ages activity increased for the first few hours under experimental conditions. Starving increased activity for the first 1–2 hr. Aphids used in experiments on a second day flew less frequently than controls which had remained on the plant the first day. AlateB. brassicaewere more active thanM. persicae: both species showed alternating periods of activity and quiescence.At light intensities between 100 and 1000 f.c. there was little difference in flight frequency, but below 100 f.c. activity declined rapidly and apparently ceased with darkness.Changes in relative humidity temporarily affected flight frequency, a change to a higher humidity retarding, a change to a lower increasing it. After adjusting to the change aphids flew readily at all humidities tested between 50 and 100% with temperatures below 80° F. (26.7°C.). The combination of high humidity and high temperature (90°F. = 32·2°C.) sometimes inhibited flight.Changes of pressure often increased activity temporarily and flight frequency was greater under fluctuating pressure than under constant pressure.It is concluded that changes in microclimate in crops are adequate to influence frequency of flight of aphids and consequently the spread of virus