The Direct-Flow Sampler is an improved balloon-borne device designed to collect particulate matter from very large volumes of stratospheric air at altitudes between 50,000 and 100,000 ft. This equipment utilizes a high-volume blower to pull air through 1 sq ft of Institute of Paper Chemistry (IPC) #1478 filter paper at rates from 400 to 800 cfm. Laboratory tests at low pressures involving aerosols of known size and density, together with intercomparison flights with other sampling devices, indicate that the Direct-Flow Sampler is a very efficient collector of submicron particles. Several vertical profiles of radioactivity over Minneapolis, Minn., that were obtained with impactor collectors and with filter samplers are shown to be in generally good agreement. Development of this sampler was supported by the Atomic Energy Commission as part of a program aimed at achieving a better understanding of the characteristics of stratospheric dust and radioactive debris, and of the mechanisms controlling fallout from the stratosphere.