The Res Gestae of Ammianus Marcellinus, a basic account of the Late Roman military history in the 4th century, including also events in the province of Isauria during 354-359, does not contain a detailed description of Isauria’s local frontier garrisons, which defended the province from the mountain and uncivilized tribes of the Isaurians. However, epigraphy (an inscription mentioning Bassidius Lauricius, a governor of Isauria) and hagiography (“The Life of St. Conon the Isaurian”) together with Ammianus Marcellinus’ account enable to extend our knowledge about the local garrisons of Isauria during that turbulent time. Based on these sources, the author considers a way of interaction between the local Isaurian garrisons and expeditionary (external) troops, who came to Isauria to more effectively defend it against the mountain Isaurians. The author tries to show how this interaction influenced the Isauria’s defence system. As the author supposes, up to 359, Isauria had no any new military detachments - a defence system of the province consisted of Isaurias’ local garrisons exclusively, which were composed as early as in the late 3d century. The garrisons were filled with Isauria’s inhabitants, but, because of their small strength and little effectiveness, they could not resist the incursions of the Isaurian tribes. Only in 359, Isauria received new expeditionary troops, who came from outside of Isauria and were subordinated to Bassidius Lauricius. Therefore, Bassidius Lauricius became a first governor of Isauria who combined two branches of power: military and civilian.