Abstract: Diplostomum is an ocular parasite known to impair vision in fish. Understanding its infection dynamics and spatial distribution in natural host populations is essential for assessing ecological risks and supporting the conservation of native fish species. In this study, we investigated Diplostomum infection in six native fish species—three schizothoracines and three loaches—from Basomtso Lake, Xizang, China. Parasites were identified using morphological and molecular approaches. We then compared infection parameters, body length trends, host frequency distributions, and spatial patterns across host species. Infection levels varied considerably among the six species. Schizothorax younghusbandi showed the highest prevalence (81.43%), with a mean abundance of 34.23±54.36 and a maximum intensity of 230 parasites per host. In contrast, prevalence in the three loach species was below 40%, and the highest mean abundance among them was 1.65±4.48—both significantly lower than that in schizothoracines. Prevalence of Diplostomum exceeded 50% across most body length groups in schizothoracines but remained below 50% in all Triplophysa groups. No significant difference in infection was detected between left and right eyes of the host (P>0.05). However, parasite site selection showed strong host specificity: Diplostomum occurred mainly in the lens of schizothoracines but was confined to the vitreous body in loaches. Parasite distribution was aggregated in all hosts, with the highest aggregation in S. younghusbandi (S²/X=86.34). These findings reveal host-dependent infection patterns and spatial distribution of Diplostomum in native fish from Basomtso Lake, offering a basis for further research on host-parasite interactions and parasite management in plateau fish populations.