Chemical, chemical-physical and biological procedures were employed to carry out a hydrobiological study of a reservoir in the 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 oligomesotrophic condition of the reservoir, with a dominance of the oligotrophic level. The analysis of the net-phytoplankton community showed the prevalence of the oligotrophic level, with a smaller tendency to the mesotrophic one. The dominant species were 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 (Planktothrix agardhii (Gom.) Anagnostidis et Komárek and Microcystis aeruginosa (Kütz.) Kützing were present, suggesting that the reservoir should be kept under accurate observation.