A study of the stonefly fauna on the left bank of the border river Una was carried out during April and May 2009, and from July to November 2010. Qualitative samples of adult Plecoptera were collected at nine different locations in the lower part of the Una River catchment area in Croatia. The aim of this study was to investigate the richness and the distribution of stoneflies along the lower reach of the Una River and its tributaries. The results are based on the identification of adult insects and only in a few cases were larvae and exuviae taken into consideration. Altogether, eleven species of stoneflies were recorded, from five families and ten different genera. Three belong to the family Nemouridae (Amphinemura, Nemoura, Protonemura), three to the family Perlodidae (Besdolus, Isoperla, Perlodes), two to the family Perlidae (Marthamea, Perla), one to the family Taeniopterygidae (Brachyptera), and one to the family Leuctridae (Leuctra). The location with the highest richness of stonefly fauna was the lower reach of Čatlan Stream, with five species recorded. The lowest richness of stonefly fauna (1 species) was recorded at the lowest Una River locality (250 m above the confluence with the Sava River in Jasenovac) and at the only spring locality (Jantalovića Spring). Despite the small number of recorded stonefly species in the study area, we found some remarkable species, which have not been recorded for quite a long time in many countries in Europe. Marthamea vitripennis was re-discovered again in Croatia after one century, together with Perla burmeisteriana and Besdolus imhoffi. These species are included on the list of 50 strictly protected Plecoptera species in Croatia. Therefore, the conservation of this important area should be one of the key concerns of environmental policies in Croatia and in Bosnia and Herzegovina.