Contracaecum sp. larvae (Nematoda: Anisakidae) parasitizing Astyanax lacustris (Lütken, 1875) (Characiformes, Characidae) and Geophagus brasiliensis (Quoy & Gaimard, 1824) (Cichliformes, Cichlidae) in the Pardo River, São Paulo State, southeast Brazil.

Astyanax lacustris (n = 26) and Geophagus brasiliensis (n = 28) were collected from two sampling sites along the Pardo River, in Botucatu, São Paulo, Brazil, and examined for parasites. Nematodes were found encysted in the intestinal serosa, heart, and fatty tissue. Morphological analysis identified the helminths as L3 larvae of Contracaecum, which was further confirmed by molecular analyses using cytochrome c oxidase subunit 2 (COX II) and large subunit ribosomal RNA (28S) genes. Phylogenetic inferences placed these larvae within a well-supported clade of Neotropical and Panamanian Contracaecum, containing sequences of Contracaecum jorgei (Argentina and Colombia) and other sequences identified as Contracaecum sp. and Contracaecum multipapillatum (Mexico, Costa Rica and Guatemala). Molecular distance analyses suggest that this Neotropical/Panamanian lineage of Contracaecum represents a single species, highlighting the fact that C. multipapillatum is currently non-monophyletic, with divergent lineages distributed across the Nearctic, Palearctic, Oriental, and Neotropical regions. Haplotype analysis supports this hypothesis with evidence of recent speciation events or ongoing gene flow within these Neotropical/Panamanian Contracaecum lineages. This new record of Contracaecum sp. larvae infecting fish in the Pardo River, Brazil, provides valuable insight into the phylogeny of this genus, which is suspected to have zoonotic potential, though no documented human infections have been reported in the Americas.