In response to a recommendation of the special NATRAN meeting of I.C.A.O. at Montreal in February 1965, the International Air Transport Association has collected flight-deck records of altimeter readings on 11000 trans-Atlantic jet flights in the period April to August 1965. These data have been processed in the Mathematics Department of the Royal Aircraft Establishment, and are presented here in graphical and tabular forms so as to be readily accessible for anyone wishing to make detailed analysis. These data do not, on their own, suffice to determine that t he vertical separation standard above FL 290 may be safely reduced. An interpretive study of these and earlier data, which is being made in an attempt to formula to conditions for reducing the vertical standard, will be reported separately.