The auxiliae or auxiliary troops, usually non-citizens enlisted from the provinces, were organized in their own units.  Infantry cohorts had 480 men in 6 centuries, as in a legion, and in fact many of the centurions were transferred legionary centurions.  The commander was a praefectus, a young man of equestrian rank appointed by a provincial governor.  A cohors sagittaria was made of archers rather than regular infantry.  A cavalry wing or ala was 512 men, made of 16 turmae (squadrons) of c. 32 men and a decurion.  Roughly half of all auxiliary units were alae, and in addition nearly half of the infantry cohorts were cohortes equitatae or "part horsed".  A cohors equitata had the usual six centuries of infantry plus four turmae of cavalry, for a total compliment of about 600 men.  This shows that cavalry formed a very significant portion of the Roman army and was highly regarded.

       In the second half of the first century AD, a few auxiliary units were increased in size to become milliaria, or "thousand-strong".  The exact organization is unclear, and the name may not have been an exact indication of the unit's size.  A cohors milliaria is thought to have had 10 centuries (or five double-strength centuries), similar to an enlarged legionary first cohort, and totalling 800 men.  An ala milliaria may have had 24 turmae for a total of 768 men--very few of these units are known to have existed, and command of one was considered a very prestigious post.  There was also the cohors equitata milliaria, composed of 10 centuries of infantry and 8 turmae of cavalry, for a total of 1056 men.  The normal auxiliary units became known as quingenaria or five-hundred-strong to differentiate them.