A cascade impactor has been designed, constructed, and evaluated for measuring the size distribution of particulates in stacks and ducts. The University of Washington cascade impactors have been used to measure the size distribution of particles emitted by a coal-fired power plant, a kraft pulp mill recovery furnace, a fluidized bed sewage sludge incinerator, and a plywood veneer drier. The University of Washington cascade impactor appears to most closely fulfill the requirements of a source test particle-size measuring system which includes isokinetic sampling capabilities, preventon of wall losses and water vapor condensation, structural ruggedness, and the ability to determine the aerosol size distribution with a minimum of effort and expense.