Praetorian Structure & Organization

Augustus set the number of Praetorian cohorts at nine following his establishment of the Principate.  By the reign of Caligula, the number of cohorts had risen to 12; this enlargement may have occured as early as Tiberius' reign at the behest of his power-hungry Prefect Sejanus, who also consolidated the previously-scattered cohorts at a new walled camp in Rome, the castra praetoria.

Vitellius is credited with enlarging the Guard further in 69 CE, to 16 cohorts.  Tacitus assigns 1,000 men to each of Vitellius' new Praetorian cohorts.  After Vespasian's victory, he cut the Guard back to nine cohorts but seems to have retained the 1,000-man cohort strength.  His son Domitian brought the Guard up to 10 cohorts.  The total strength of 10,000 Guardsmen remained fairly constant through the remainder of its existence.

The Guard Infantry

There is longstanding controversy over the exact size of the Praetorian cohorts at their inception under Augustus.  While Durry (and Peter Connolly after him) maintained that 500 was the initial size of each cohort, Durry's contemporary Passerini argued that 1,000 men was the actual strength, and Campbell supports the latter figure.

Assuming the more conservative figure of 500 men per cohort, Augustus'  nine cohort-strong Praetorian Guard would have had about 4,500 men at its inception, not including the Guard cavalry of which little is known.  Three infantry cohorts totaling 1,500 men were stationed in Rome, while the other six cohorts with 3,000 men were stationed abroad in Italy.

Each cohort of about 500 men was commanded by a tribunus praetorio (Praetorian tribune) with a centurio (centurion) commanding each of six subordinate centuries of 80 men each.

The Guard Cavalry

It can be inferred from epigraphic evidence that, like the legions they derived from, the Praetorian Guard did include a small contingent of cavalry, but the exact number and disposition are unclear.  It is also unknown whether the mounted arm was expected to operate in the field as actual cavalry, or more as infantry mounted to keep up with the mounted imperial retinue.

Following Michael Speidel's argument in Riding for Caesar, the strength of the Guard cavalry arm would have varied from 400 to 1000 troopers.