Unlike other informal sites, fossil parks provide visitors collecting opportunities that result in ownership of a small number of fossils. In 2003, we investigated the first three identified U.S. fossil parks at Hamburg, New York; Sylvania, Ohio; and Rockford, Iowa. Case study analyses determined the opportunities to learn geobiology at each site. Data collection proceeded through lived learning experiences, and included field notes, photographic records, informal conversations with park participants, brochures, and on-site signage. Through constant comparative methods, six variable categories converged for fossil park development: (1) informative previsit Web site, (2) authentic collecting in situ, (3) authentic collecting tools, (4) accessibility, (5) fossil identification, and (6) visitor education. These variables were optimized in a model of fossil park design. In 2005, fossil parks at Sharonville, Ohio, and Fossil, Oregon, were investigated in phase 2 of our study, and in 2006, our third case study researched fossil parks in Aurora, North Carolina, and Republic, Washington. Analysis of the seven U.S. fossil park data sets resulted in the emergence of key variables that affected the visitors' opportunities to learn geobiology concepts at fossil parks: (1) authenticity of experience, (2) age of fossils, (3) fossil-collection training and facilities, (4) availability of on-site paleontological mentors, (5) fossil identification via signage and brochures, (6) site organization and wayfinding signs, and (7) accessibility of site, including safety. The seven U.S. fossil parks were ranked against these variables according to their effectiveness as informal science education sites. We conclude that fossil parks can provide valuable informal geobiology education that can contribute to the public's geobiological literacy.