The leptophlebiid mayfly genus Ulmeritoides is composed of nine species, three of which have been recorded in Brazil: U. patagianus, U. uruguayensis, and U. misionensis. In the present paper, a new species of this genus, Ulmeritoides oepa sp.nov., is described and illustrated based on fifteen larvae collected in Uraricoera River, state of Roraima, northern Brazil. The species was compared to U. misionensis, U. tifferae, and U. guanacaste, the other species of Ulmeritoides known from the larval stage. U. oepa sp.nov. can be distinguished from these species by the following combination of characteristics: 1) medial denticle on anteromedian emargination of labrum much larger than others; 2) anterior tibiae slightly stained black toward the apex; 3) medial femora with few dorsal spines, similar to short thick needles; 4) hind femora with numerous dorsal spines, similar to short thick needles; 5) medial femora with no medial black markings, apical brown markings. The new species seems to be more closely related to U. misionensis, another species in which the anteromedian emargination of the labrum has a median denticle much larger than the others. The studied specimens were deposited in the Invertebrates Collection of the Amazonian Research National Institute, Manaus, state of Amazonas, Brazil, and in the Entomological Collection of the Department of Zoology, Federal University of Rio Janeiro, Rio de Janeiro, state of Rio de Janeiro, Brazil.
Keywords: Insecta, Ephemeroptera, Leptophlebiidae, Atalophlebiinae, Ulmeritoides, taxonomy, Brazil