Fledermausquartier Brauereikeller Frankfurt, Protected bat habitat in Frankfurt (Oder), Germany

The Fledermausquartier Brauereikeller Frankfurt (Oder) is a former brewery cellar system beneath the city that now protects bats. The underground network consists of several cellar rooms with vaults and maintains stable conditions during the cold months for the animals.

The Ostquell-Brauerei, a 19th-century brewery on the Bahnhofsberg, was shut down in 1950. In 1987 the first count found 250 bats in the cellar; after conservationists sealed the ruin, numbers rose to as many as 2,500 in the late 1990s. A citizens' initiative formed in 1989 against the planned demolition, and the site became a nature reserve in January 2003, bought by the Euronatur Foundation with the BfN and Brandenburg.

Local citizens formed an initiative in 1989 to prevent the demolition of the structure and establish protective measures for the bat population.

The ruin is fenced and closed to the public; the cellar is secured by steel doors and checked regularly against unauthorised entry, so there is no guided tour. The bat quarters lie in seven vaulted cellars up to 8 m high, at about 5 °C and nearly 100 % humidity. Observation is limited to the exterior on Bahnhofsberg.

The Greater Mouse-eared Bat is the most numerous species here, and the ruin is currently Germany's largest known winter roost for it. Bats ringed in Frankfurt have been found as far as the Müritz and the Czech Republic, and researchers also ring bats inside the ruin. Most overwintering mouse-eared bats are thought to come from western Poland.