The Hadrianic fort at Maryport in Cumbria is well known for the large series of altars found in its vicinity, including twenty dedicated to Jupiter Optimus Maximus (RIB 815–31, 833–35) and one to Jupiter Augustus (RIB 814). Of these, sixteen were dedicated by cohors i Hispanorum equitata and its successive commanders; they are thought to be annual dedications made either on 3 January or on the anniversary of the emperors succession. Four of these altars were dedicated by M. Maenius Agrippa, and four by C. Caballius Priscus, both of whom are recorded as holding the rank of tribune; four other commanders have the rank of prefect, save one who might have been a centurion in temporary command as praepositus. If the dedications are annual, a total of fourteen or fifteen years is involved. As prefects normally commanded cohorts five hundred strong while milliary cohorts were commanded by tribunes, it has often been suggested that cohors i Hispanorum during Hadrian's reign had been doubled in strength for about eight years, only later to be reduced again to its original size. The cohort had been in Britain at least since the Flavian period, when it is recorded at Ardoch (RIB 2213); but its expansion to milliary strength under Hadrian, ifit occurred at all, which in this paper I suggest it did not, would probably have happened c. A.D. 125, because the newly built fort at Maryport at c. 1.9 ha is too large to have been planned for a quingenary unit. The forts of these units normally averaged c. 1.5 ha, whereas Maryport approaches the average for milliary cohorts of c. 2 ha. The reduction of the cohort back to its original strength, it has been argued, would have taken place in the early 130s, when a detachment may have been sent to serve in the Jewish War, which broke out in 132.