The mouse weighs about the same as a 20p piece
The hunt is on for Britain's smallest rodent as Suffolk Wildlife Trust launches a survey to identify the numbers of harvest mice in the county.
The work will involve searching for nests and recording land use.
The harvest mouse has suffered a 70% decline in numbers since the 1970s due to land use changes and habitat loss.
The Trust said it would use the information gathered to deliver pro-active, site-based landowner advice and training on how to encourage the mice.
Martha Cowell who is leading on the project said: "Our work on previous wildlife projects has shown that positive action by landowners can turn around a critical situation for dwindling populations."
The harvest mouse weighs about the same as a 20p piece and is found in a wide range of habitats including long grass, cereal crops, hedgerows, bramble patches and reedbeds.