ABSTRACT: Moderately well-preserved radiolarians are described from two measured sections of the Reef Trail Member of the Bell Canyon Formation (Middle Permian), exposed in the Patterson Hills, Guadalupe Mountains National Park, West Texas. The Reef Trail Member is the youngest member of a series of basinal marine carbonates deposited in the Delaware basin prior to the deposition of the Castile Formation evaporites, and represents the uppermost Guadalupian (upper Capitanian). The Guadalupian-Lopingian boundary is presumed to occur at or just above the top of the Reef Trail-Castile contact because of the presence of C. postbitteri hongshuiensis ~ 2m below the top of the Reef Trail Member in sections in the Patterson Hills. A total of 30 genera containing 51 species were recovered from the Reef Trail Member, belonging to the orders Albaillellaria, Latentifistularia, Spumellaria, and Entactinaria. The following new taxa are described: Raphidociclicus scutum, Camptoalatus volaticus, Pseudoalbaillella delawarensis, Astroentactinia porosa, Stigmosphaerostylus favusa, Polyedroentactinia quadrata, Praedeflandrella firmata, P. prolata, Copicyntra spinosa, Copicyntra irregulata, and Paracopicyntra puncta, and Copicyntroides nazarovi. The following taxa are emended: Raciditor scalae (Caridroit and De Wever 1986), and Klaengspongus Sashida 2000b. Near the top of the Reef Trail Member, there are marked fluctuations in the relative abundances of albaillellarian and latentifistularian species. Fluctuations are possibly controlled by paleoenvironmental changes associated with the onset of the end-Guadalupian extinction event. Biostratigraphically, the fauna most closely correlates with the widely recognized F. bipartitus – F. charveti Zone sensu Caridroit, which we now consider to be uppermost Guadalupian, not Lopingian, and to the Guadalupian F. charveti Zone of Sun and Xia 2006 from oceanic facies in China. Both older and younger biostratigraphic markers also occur in the Reef Trail fauna, including Pseudoalbaillella longtanensis and A. yamakitai, requiring a reevaluation of the reliability of these albaillellarian taxa in biostratigraphy.