The distinguished oldfield mouse is a Thomasomys recorded in the Peruvian Andes. Its original description notes a dark stripe on the back and dark marks on the feet; a diet study of montane-forest specimens found mostly plant matter in their stomachs.
The original description calls it medium-sized, with a dark stripe along the back and dark patches on the upper surfaces of the feet. The type specimen was a male from Torontoy, Peru.
Stomach contents from six animals collected in Huánuco montane forests averaged 89.2% plant matter, with monocotyledons especially common.
An extinction-risk assessment has not been added yet.