Assessment of quality of the Apennine rivers (Italy) using the diatom indices: the River Foglia. Within the framework of a monitoring program assessing the biological quality of the central Italian watercourses, the benthic diatoms of the river Foglia (Marches region) were studied for the first time. Nine sites were selected upstream and downstream of the principal anthropological activities, which occur along all the catchment's area. Two samplings were carried out: the first in October 2004 and the second in May 2005. In total, 145 taxa were observed, the large majority of which clearly indicate both organic and mineral pollution, especially evident in the lower reaches of the river and in particular during the period of low water conditions. The biological quality of the watercourse was assessed using the Diatom-based Eutrophication/Pollution Index or EPI-D, developed for Italian rivers. This index is based, above all, on the diatom sensitivity to nutrients, organic matter and salt concentration, in particular the chlorides. It showed the bad water quality of the river Foglia as well as an excellent correlation with other European diatom indices and the chemical parameters.