A diaphragm pump used to boost the intake pressure of a three-stage high-pressure air compressor for collecting large air samples during aircraft flights has been developed. The pump consists of a large (17 L) spherical body divided into two chambers by means of a thin neoprene diaphragm. The pressure drop generated across an air intake and air exhaust mounted outside the aircraft drives sample air into this spherical body while the diaphragm is displaced completely to align the inside of the pump body. The air thus collected is subsequently forced into the inlet of a high-pressure compressor by letting aircraft cabin air fill the pump body from the other side of the diaphragm. The pumping cycle is controlled by the position of the diaphragm and is self-sustaining. The pressure at the intake of the high-pressure compressor is thus maintained close to that of the cabin pressure, which results in a three- to fourfold pressure boost. In this way the pumping speed of the high-pressure compressor is enhanced considerably, reducing sampling time and possible contamination of the air sampled. Details of construction, laboratory tests, and aircraft performance are given, with special attention to the problem of collecting air in the lower stratosphere for the isotopic analysis of carbon monoxide.