The structure of hurricane Cleo (1958) is presented, based on observations of wind, temperature, pressure, humidity, clouds, and precipitation obtained from three research reconnaissance aircraft of the National Hurricane Research Project. Insofar as possible, the data are analyzed in terms of departures from the mean tropical atmosphere; and the physical processes which produce these departures are discussed. The central eye region of the hurricane emerges as the seat of the most important contributions to the structure of the entire storm, a result anticipated by Wexler from what may be termed the first research reconnaissance of a hurricane in 1944.