This five-hundred strong infantry unit was recruited from the Delmatae tribesmen inhabiting the Adriatic coast of modern Croatia. This regiment is attested on a number of inscriptions at High Rochester in Northumberland, forward of Hadrian's Wall, two of them dated to the reign of Antoninus Pius in the period AD138-61, although the unit is well attested in the fort at Maryport in Cumbria during the same period. It is possible that a tombstone recovered from the High Rochester fort may instead be attributed to Cohors I Dacorum.

Evidence for the Cohort in Britain

1. Burn 100; CIL XVI.65; military diploma dated July 17th AD122.
2. CIL VII.1195 privilegia militvm; dated: September 16th AD124.
3. L'Année Épigraphique 1997.1001 diploma dated 27th February AD158.
4. Chesters (RIB 1496a).
5. Maryport (RIB 810 altar; 847 altar; 831 altar AD138-161; 850 AD138-161).
6. High Rochester (RIB 1289 tombstone poss. Coh I Dacorum).