As experiments have shown that pollen capture by the standard Durham sampler is determined by several factors in addition to the concentration of pollen in the air, a study was conducted to determine what type of sampler would be the most acceptable replacement. Many types of samplers, including deposition surfaces, stationary and rotating impactors, suction, and filter devices, were tested in a wind tunnel and in the field under various meteorological conditions. Considerations of accuracy of sample, use under varying weather conditions, simplicity of operation, case of counting, conversion of catch to a volumetric basis, and cost led to the development of the rotoslide, a rotating impactor which takes samples of pollen and other spores on the edge of standard glass microscope slides. Staining and other preparation of samples for study are similar to the methods in routine use. A special inexpensive holder positions the slides under a microscope. Identification and counting are done at 100\texttimes or at higher magnifications if desired. The rotoslide sampler is essentially independent of wind speed and direction and has a collection efficiency of approximately 64 per cent for particles in the size range of pollen. It samples approximately 60 L. per minute, but for long-period samples it may be operated intermittently to prevent overloading.