The Bureau of Mines conducted a flammability study of plastic ventilation ducts. Six rigid 16-in-diam fiberglass-reinforced plastic ducts from five manufacturers were examined using laboratory- scale, large-scale, and full-scale fire tests. The laboratory-scale tests included the American Society for Testing and Materials e162 radiant panel test, the e2863 oxygen index test, and the standardized small-scale flammability test (s3ft) developed by the Mine Safety and Health Administration, U.S. Department of Labor, and replicated by the Bureau of Mines. Ten-foot lengths of the same ducts were also subjected to a large-scale vertical fire test, and 20- to 30-ft lengths were tested in a full-scale in-mine fire test at the Bureau's Lake Lynn Laboratory Mine. The data from the laboratory-scale radiant panel test and oxygen index test indicated that the ducts were fire-resistant. However, five of the six ducts failed the s3ft and vertical fire tests, and four of the ducts failed the full-scale in-mine tests with flame propagation rates that ranged from 3 to 10 ft/min. The results indicate the need for improved fire-resistant plastic ducts. The good agreement between the s3ft results and the vertical and full-scale tests show that the s3ft provides a good indication of the flammability of fiberglass- reinforced plastic ducting under realistic fire conditions.