Fossil-bearing concretions found in the Francis Creek Shale of the Carbondale Formation near Mazon Creek, Illinois, have attracted international attention. The abundant and diverse biota of this Pennsylvanian delta complex has been studied for almost 150 years, resulting in many taxonomic descriptions. Numerous Mazon Creek fossils in private collections have been designated as type, figured, or referred specimens. The locations of privately owned specimens are less permanent than those of institutional collections. -- A review of 93 scientific articles on Mazon Creek fossils published between 1844 and 1987 indicates that 426 specimens belong to 69 private collectors. Because published (type, figured, and referred) specimens form the foundation of paleontology, knowing the current locations of these specimens is important. This paper, based on contacts with most of the 69 private collectors, provides current information on locations of their collections Includes bibliographical references (p. 23-26) Fossil-bearing concretions found in the Francis Creek Shale of the Carbondale Formation near Mazon Creek, Illinois, have attracted international attention. The abundant and diverse biota of this Pennsylvanian delta complex has been studied for almost 150 years, resulting in many taxonomic descriptions. Numerous Mazon Creek fossils in private collections have been designated as type, figured, or referred specimens. The locations of privately owned specimens are less permanent than those of institutional collections. -- A review of 93 scientific articles on Mazon Creek fossils published between 1844 and 1987 indicates that 426 specimens belong to 69 private collectors. Because published (type, figured, and referred) specimens form the foundation of paleontology, knowing the current locations of these specimens is important. This paper, based on contacts with most of the 69 private collectors, provides current information on locations of their collections Fieldiana series has been published as Geological Series by Field Columbian Museum (1895-1909) and Field Museum of Natural History (1909-1943), and as Fieldiana: Geology by Chicago Natural History Museum (1945-1966) and Field Museum of Natural History (1966-1978). Fieldiana Geology New Series No. 1 began June 29, 1979