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Understanding the relative strength of ecology and phylogeny in shaping parasite communities can inform parasite control and wildlife conservation initiatives while contributing to the study of host species evolution. We tested the relative strengths of phylogeny and ecology in driving parasite community structure in a host whose ecology diverges significantly from that of its closest phylogenetic relatives. We characterized the gastrointestinal (GI) parasite community of wild geladas (Theropithecus gelada), primates that are closely related to baboons but specialized to graminovory in the Ethiopian Highlands. Geladas exhibited very constrained GI parasite communities: only two genera (Oesophagostomum and Trichostrongylus) were identified across 303 samples. This is far below the diversity reported for baboons (Papio spp.) and at the low end of the range of domestic grazers (e.g., Bos taurus, Ovis aries) inhabiting the same region and ecological niche. Using deep amplicon sequencing, we identified 15 amplicon sequence variants (ASVs) within the two genera, seven of which matched to Oesophagostomum sp., seven to Trichostrongylus sp., and one to T. vitrinus. Population was an important predictor of ASV richness. Geladas in the most ecologically disturbed area of the national park exhibited ~4x higher ASV richness than geladas at a less disturbed location within the park. In this system, ecology was a stronger predictor of parasite community structure than phylogeny, with geladas sharing more elements of their parasite communities with other grazers in the same area than with closely related sister taxa.