ABSTRACT . – Eight watercourses in North-Central Sardinia were studied between 2008 and 2009 to ascertain their environmental quality and to gain insights into the requirements for their proper management. In addition to physicochemical and microbiological parameters, epilithic diatoms were analyzed at 15 stations. Data on the diatom community of each sample were used to calculate the diatom-based eutrophication/pollution index (EPI-D) and the biological diatom index (IBD). In the 21 collected samples, 153 taxa belonging to 36 genera were identified, but the diatom flora also included species that could not be determined with certainty. Most of the observed diatoms are considered cosmopolitan and were common to various sites, with Cocconeis placentula var. pseudolineata and Diadesmis confervacea among the most frequent and sometimes the most abundant taxa. Platessa hustedtii, which like Diadesmis confervacea is considered a tropical species, was recorded for the first time in Sardinia. While these species are not considered by the EPI-D proposed for use in Italian watercourses, their integration will improve the usefulness of this index in the assessment of islands of Mediterranean typology. The applicability of the EPI-D and IBD was confirmed with water quality at nearly all the considered stations ranging from mediocre to bad according to both indices. Moreover, these results were supported by physicochemical and microbiological variables and were consistent with the typology of the investigated environments.