The neotype of Ecdyonurus nigrescens (Klapálek, 1908) has been designated, basing on male imago reared from larva, collected in the Chornohora Range (the Ukrainian Carpathians). The male imago of this species is redescribed; male subimago, female imago and subimago, larvae and the structure of egg chorion are described and illustrated for the first time. Critical diagnostic characters distinguishing this species from other representatives of the Ecdyonurus helveticus species-group are discussed. The type locality is defined in accordance with the Article 76.3 and Recommendation 76A of ICZN as follows Ukraine: Zakarpattia Region, Carpathian Biosphere Reserve, the Chornohora Range, Polonyna Brebeneska district, mountain valley between SW slope of Gutyn Tomnatek Mt. and NW slope of Brebeneskul Mt., upper section of the Brebeneskul stream (left-bank tributary of the Hoverla stream, Tysa river-basin), 1450 m. a.s.l. The data on the original syntype series and type locality are discussed in detail. All known specimens of E. nigrescens collected by J. Dziędzielewicz between 1908 and 1910 are observed and reidentified. The data on distribution, ecology and life cycle of E. nigrescens are presented.