After taking samples of aphids regularly in the air over southern England for 3 years, Johnson (1951) wrote 'it is evident that there is a general tendency for aerial populations up to 2000 ft (610 m) to build up during the day and to decline at night, often leaving the air virtually clear of aphids in the early morning'. Reviewing the increasing evidence in 1965, Taylor still found 'All the evidence showed that, over southern England, the upper air cleared of aphids, and other insects, each night and a new aerial population built up each day'. Glick's (1939) catches, collected by aeroplane over Louisiana in the southern United States, remained anomalous, for he found 'more insects-at night, in proportion to the amount of time flown, than during the day'. If insects continue to fly through the night, or resume flight on successive days as locusts do, they may migrate 1000 miles or more. Experimental evidence (Cockbain 1961a) suggests that few aphids will fly for long on the day after an exhausting full day's flight. Hence, the migratory ambit of an aphid species is probably limited by the ability of its individuals to fly through the night. Evidence from experiments suggests that night flight by aphids is unlikely, yet from circumstantial evidence of long-distance migration of the greenbug (Schizaphis agrostis Hille Ris Lambers = Toxoptera graminum Rondani) in the Central Plains of North America, overnight flight seems to be essential (Taylor 1965). We therefore sampled aphids by day and night at 2000 ft (610 m) above ground over eastern Kansas, immediately in the path of the supposed migration, in low-level jet streams and compared aerial aphid densities directly with similar measurements made in earlier years over southern England. The aphid community of the maritime aerial environment at high altitudes over England is thought to be maintained by aphids being constantly recirculated and constantly replenished by recruitment of new aphids from the ground, few remaining aloft for long and none overnight. If aphids are aloft over Kansas at night, this hypothesis needs to be modified, or extended to relate to the different, continental, aerial environment.