Dorsal-spined protostrongylid nematode larvae (Metastrongyloidea: Protostrongylidae) were recovered from the feces of the endangered pampas deer (Ozotocerus bezoarticus celer) in Campos del Tuyú Wildlife Reserve, Bahia Samborombón, Argentina. Partial DNA sequences from the large subunit ribosomal RNA (LSU rRNA) gene and from the second internal transcribed spacer region (ITS2) were amplified, cloned, sequenced, and compared to those of other nematodes. Nucleotide alignment and phylogenetic analysis of the sequences indicate that this protostrongylid nematode is most closely related to Parelaphostrongylus spp. as inferred from the LSU rRNA sequence analysis. Analysis of the ITS2 spacer indicated that the pampas deer protostrongylid is nested in a clade containing Parelaphostrongylus and Elaphostrongylus spp. These sequences differed considerably from those of other protostrongylid nematodes, and were most similar to those of Parelaphostrongylus spp. and Elaphostrongylus spp. in spite of clear variability from both genera. These results suggest that the protostrongylid from pampas deer is an undescribed nematode that likely belongs in the subfamily Elaphostrongylinae.