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Auriculostoma is a genus of digenean (Trematoda) whose adults are parasites of Neotropical freshwater fi shes. We describe Auriculostoma ocloya n. sp. using morphological and molecular tools, and we elucidated its life cycle, the fi rst known of a species of this genus. The fi rst intermediate host is the bivalve Pisidium ocloya, the second intermediate host is the amphipod Hyalella sp., and the defi nite hosts are siluriform fi shes. The adult presents a single pair of muscular lobes on either side of the oral sucker with a broad base, stretching from ventrolateral to dorsolateral side, a structure also present in the rest of species of the genus. Nevertheless, the new species differs from all congeners by the combination of several traits, and mainly because on the dorsal side "free" ends of the lobes are absent because they are fused. This is the fi rst study to provide sequence data on larval and adult stages of a species of Auriculostoma. Our phylogenetic analysis demonstrated its basal position among species of the genus. Therefore, integrative morphological, molecular, and life cycle data on other South American species of the genus, would contribute to reveal more patterns in the allocreadiid systematics.