Arger Fen is a 49.7-hectare (123-acre) biological Site of Special Scientific Interest (SSSI) south-east of Sudbury in Suffolk, England.

[9] The underlying geology is a mixture of sand and gravel banks and clay soils, producing a mix of habitat types, including wet fen type habitats at lower levels and dry grasslands on acidic soils on hill tops.

[9] It is one of only two known areas of ancient woodland in Eastern England which feature wild cherry (Prunus avium).

[11] There are onsite car parking facilities as well as two-way marked trails, including areas of board walk.

[7] The trust has attempted to encourage the growth of the dormouse population, partly by expanding the area of land it owns at Arger Fen.