Nemoura kozari sp. n. is described on the basis of morphology of male adults collected in the Eastern Carpathians. The new species is classified as a member of the Nemoura marginata species group. It was found in medium elevations of the Krasna Mts, Ukraine, and the Rodna Mts, Romania, inhabiting slowly flowing, small brooks. New diagnosis of the marginata group and enumeration of its members are provided.