Chemical, chemical-physical and biological procedures were employed to carry out a hydrobiological study of a reservoir inthe central Apennines (Italy), designed for the supply of drinking and irrigation water, and tourist recreation facilities.Potentiometric, volumetric, spectrophotometric, and chromatographic techniques were used for the chemical-physical analyses,while the OECD method was adopted to assess the trophic level. The phosphorus and chlorophyll values evidenced the oligomesotrophiccondition of the reservoir, with a dominance of the oligotrophic level. The analysis of the net-phytoplankton communityshowed the prevalence of the oligotrophic level, with a smaller tendency to the mesotrophic one. The dominant specieswere Dinobryon divergens Imhof and D. sertularia Ehrenb. among the Chrysophyceae, and Ceratium hirundinella (O.F. Müll.)Dujardin and Peridinium cinctum Ehrenb. among the Dinophyceae. However, some potentially dangerous inocula (Planktothrixagardhii (Gom.) Anagnostidis et Komárek and Microcystis aeruginosa (Kütz.) Kützing were present, suggesting that the reservoirshould be kept under accurate observation.