Devon Wildlife Trust is taking High Court action over a council decision to build houses on a site where rare bats breed.
Teignbridge District Council approved plans for 230 houses to be built 170 metres from land where greater horseshoe bats congregate to give birth and raise their young.
The case is due to be heard at Bristol later today.
Researchers have found a sharp drop in the numbers of one of Britain's rarest mammals.
It was thought there were greater numbers of grey long eared bats but a survey has found there are just 1,000 remaining.
Jacquie Bird reports from a roosting site in Devon:
Unique footage of baby bats learning to fly is being shown to visitors at Hestercombe House near Taunton. The attraction used CCTV cameras to film the lesser horseshoe bats without disturbing them, as well as special infra red equipment to capture their night flights.
Watch David Woodland's story for more details.