Palingenia longicauda (OLIVIER, 1791), a well-known species of mayfly belongsto the most critically endangered species of the Ephemeroptera in Europe. It represents the Pontic faunistic element of the expansive type originally distributed in almost all great and numerous middle-sized European rivers in Austria, Belgium, Bulgaria, Czech Republic, France, Germany, Hungary, Macedonia, Poland, Rumania, Slovakia, Ukraine and Yugoslavia. At present, the species is extinct or at least missing at most of its original area except for relatively small refugia in the Tisza basin in Hungary and probably also Slovakia and Ukraine. Distribution of P. longicauda is discussed and a new subarea in the Danube delta in Rumania (locality near Murighiol, stretch of the Bratul Sfintu Gheoghe) is documented for the first time. The nature of a new occurrence is discussed – it might represent either a new, still unknown refuge and/or recovery of the original population(s) caused by downstream drift