Floristic and ecological features of aquatic bryophytes, and main abiotic factors, were studied in 23 mountain streams located in the Western Alps and Central Apennines (Italy). At 46 stations, a total of 36 taxa were collected. Most species showed both low occurrence (≤ 2 records) and low cover respect to sampled area (< 4%). However, in over 50% of stations, bryophyte communities with at least three species and cover over 10% were recorded. Platyhypnidium riparioides, Fontinalis antipyretica subsp. antipyretica and Hygrohypnum luridum were the most common and abundant species. Other species, more rarely recorded, have limited distribution in the Mediterranean region (Blindia caespiticia) and Italy (Hygrohypnum duriusculum). Some species show specific ecological preferences. In particular, Hygrohypnum luridum and Palustriella falcata are mostly linked to turbulent and cold waters (11.0–16.0°C), neutro-alkaline (7.3–8.4), with low conductivity (< 160 µS) and phosphates (< 0.01 mg/l) values. Brachythecium rivulare, Platyhypnidium riparioides and Fontinalis antipyretica subsp. antipyretica occur abundantly in less turbulent and less cold (15.3–18.2°C) waters, shaded, alkaline (8.3–8.6), with moderate conductivity (330–440 µS), and higher values of phosphates (0.06–0.09 mg/l). This study provides new floristic and ecological data on bryophyte communities occurring in mountain streams. It can also represent a useful contribution for biomonitoring activities of these habitats, often understudied floristically, where bryophytes are a significant part of the aquatic macroflora.