Urocleidoides ectoparasites are mainly found on fish of the neotropical regions. Although molecular research on monogeneans is available, no genetic data exist characterizing species in the Urocleidoides genus. Some DNA sequences have been efficacious in systematic studies and in the reconstruction of phylogenies of fish parasites. Relevant roles have been given to the sequence of the mitochondrial gene of cytochrome c oxidase I (COI). This study characterized COI sequences of the parasites Urocleidoides malabaricusi and U. cuiabai in trahira fish Hoplias aff. malabaricus of the flood plain of the Upper River Paraná, Brazil. The two species under analysis were distinguished by sequencing and analysing a 420-bp fragment of the COI gene, which suggested the existence of the cryptic species U. malabaricusi.