The fibrous zeolite mineral erionite is a concern due to its potential for causing lung disease in humans. Studies have shown that exposure to altered volcanic bedrock containing erionite may be the explanation for a high lung disease rate throughout regions of the world (Metintas et al., 1999). Erionite was reported by Forsman (1986) as occurring in tuffaceous rock units of the Arikaree Formation in the Killdeer Mountains of western North Dakota. Rock and soil samples were collected where zeolite minerals are known or suspected to be present including North and South Killdeer Mountains in Dunn County and West and East Rainy Buttes and White Butte in Slope County. Analysis and identification was carried out using XRD, SEM, and EMPA. Zeolitic material was confirmed in units for both North and South Killdeer Mountains. The chemical compositions of the fibers resulted in a majority being classified as offretite rather than erionite.