Atelognathus patagonicus is an endangered leptodactylid frog endemic to a small region in and around Laguna Blanca National Park in northern Patagonia, Argentina. All of the lakes and small ponds of the region (except Laguna Blanca itself) contain A. patagonicus and in all but one of these lakes the species shows clinical signs of a previously undiagnosed disease, the characteristics of which suggested a ranavirus. We collected symptomatic and asymptomatic A. patagonicus frogs and tadpoles from 4 small lakes and analyzed tissues for ranavirus and the chytrid fungus Batrachochytrium dendrobatidis using PCR amplification of pathogen DNA. Of the 32 specimens tested, 25 were positive for ranavirus major capsid protein (MCP). Sequence alignments of the ranavirus MCP from these specimens showed 100% similarity with published FV3 and FV3-like viruses from anurans, 98 to 99% similarity with Bohle iridovirus, and 95% similarity with Ambystoma tigrinum virus (ATV) and Regina ranavirus (RRV). A search of the NCBI Blast nucleotide database using the 500 base pair MCP sequence obtained from these samples did not suggest any homology to any other pathogen. In addition, 1 sample (3 pooled individuals) from 1 lake tested positive for B. dendrobatidis. The clinical signs observed primarily in late-stage tadpoles and recent metamorphs, which have reoccurred each year since at least 2001, are consistent with ranaviral disease, but until histopathology of diseased individuals is carried out, chytridiomycosis or other diseases cannot be ruled out.