Patagonian maras, rodents endemic to South America, are classified as a near-threatened species. Various factors affect their health including parasitic diseases. The aim of this study was to perform morphometric, molecular and phylogenetic characterisation of one such parasitic disease agent, the nematode Graphidioides affinis, specimens of which were found in captive Patagonian maras.In March 2023, 18 Patagonian maras kept at the Sofia Zoo in Bulgaria were investigated with the use of coprological methods. Following the investigation, the animals were dewormed with the use of albendazole. Dead adult nematodes found in the faeces of dewormed maras were collected and preserved in 70% ethanol, and morphometrically, molecularly and phylogenetically analysed.The morphometric analyses confirmed the nematodes to be Graphidioides affinis. The partial nucleotide sequences of the small subunit ribosomal rDNA (SSU), the internal transcribe spacer 2 (ITS2) and the large subunit ribosomal DNA (LSU) of G. affinis were obtained. These are the first available nucleotide sequences of this parasite. The phylogenetic analyses of the species showed its distinctiveness in comparison to other gastrointestinal nematodes, as it was grouped separately.The Patagonian maras kept in a European zoo retained their original parasitofauna which are related to South America.