Last year 140 lesser horseshoe bats were counted at the disused mine
A group of rare lesser horseshoe bats, which roost in a disused Gloucestershire mine, will benefit from plans to give the colony more space.
The old limestone mines in Quarry Wood at Sherborne have become home to a nationally important colony.
Now plans, costing £37,5000, have been unveiled to create a second roost site nearby.
Each winter the bats are counted and the numbers have steadily increased from 89 bats in 2004, to 140 in 2009.
The bats are cared for by The National Trust with help and advice from the Gloucestershire Bat Group.
"More work needs to be done to protect this important roost site," said the National Trust's head warden at Sherborne, Nathalie Bradley.
"We are concerned that, with the colony expanding, the bats could become overcrowded so the creation of a second roost site in one of the other tunnels would provide a backup to the main site, and allow the existing colony to expand."
The work has been made possible by funding from the Enriching Nature programme from SITA Trust.
"The grant enables us to make a number of important improvements to the bat roost, including commissioning a microgravity and radar survey to locate new shafts for a suitable new entrance and an air flow shaft," added Nathalie.
The mines were worked on a large scale until the 1820s, and continued to be mined until World War I.
All but one of the mines have since been back filled with rubble.
In recent years the National Trust and Gloucestershire Bat Group have carried out work to stabilise the existing entrance and prop up the crumbling roof structure.
"We will also improve the existing entrance by fitting a grille and removing tree roots and any trees in danger of collapsing near the mine face, along with other planting nearby to encourage feeding areas," said Nathalie Bradley.