ABSTRACT: Xenobiotic mixtures in aquatic ecosystems may compromise fish health by inducing oxidative stress, decreasing resistance, and fostering susceptibility to pathogens such as helminths. Lipid peroxidation (LPO) is one of the most commonly used methods to evaluate oxidative stress. The present study examines the potential correlations between LPO in liver, muscle, and gill and the presence of helminths in Astyanax aeneus from a subtropical river in Mexico. Five helminth species were identified: the trematodes Uvulifer ambloplitis, a subdermal larval parasite, and Clinostomum complanatum, which infected the base of the gills; the nematodes Contracaecum sp. and Spiroxys sp.; and the acanthocephalan Polymorphus sp. The latter 3 species occurred in the posterior half of the intestine. Total helminth abundance was highest in April and July. Helminth species richness increased along a gradient from the headwaters to the middle reaches of the river. An integrated biomarker response for LPO (IBR-LPO) was assessed in the 3 tissues analyzed. IBR-LPO peaked in July. A direct correlation was found between gill and muscle LPO and mean abundance of Clinostomum complanatum, Contracaecum sp., and orthophosphate values, as well as between liver LPO and mean abundance of Polymorphus sp., whereas no relationship was found between LPO and U. ambloplitis or Spiroxys sp. This research revealed an association of both orthophosphate values and the presence or abundance of parasites with oxidative stress levels in A. aeneus, without any causal association, but rather as a result of synergistic processes associated with these variables.