Elachista cahorsensis is a moth of the Elachistidae family. It is found from Great Britain, Ireland and Germany to the Pyrenees. The wingspan is about 8 millimetres (0.31 in). The larvae feed on Festuca rubra. They mine the leaves of their host plant. The mine starts as a small gallery close to the base of the leaf. It ascends to just under the tip of the leaf, and finally widens to the full width of the blade. The frass is deposited in the basal part of the mine. Larvae can be found in July and again from September to November. They are pale grey with a semi-transparent, reddish brown head.