Distribution of Thremma anomalum was well documented by the late 1980s. However, macrozoobenthos research within Serbian watercourses between 1989 and 2010 reveal a shift of the western boundary of the distribution. Recent research in western Serbia and northern Montenegro has shown no trace of this species, in spite of its previous presence. In Bosnia and Herzegovina, the species was found in only one of ten former localities, implying that the species might entirely vanish from zoographic region 5 in the near future. On the other hand, the species was found at 17 localities in eastern, southeastern and southern Serbia, sections of the area where the species was not recorded earlier. This research has shown that T. anomalum is not restricted to cold waters and builds specific biocenoses characterized by dominance of Trichoptera and Gammaridae.