It is not until the time of Darius Hystaspis that we have trustworthy evidence of their existence as a distinct people.

In the inscriptions of this king we find their country included under the name of Parthva or Parthwa among the provinces of the Persian Empire, joined in two places with Sarangia, Aria, Chorasmia, Bactria, and Sogdiana, and in a third with these same countries and Sagartia.

We find, moreover, an account of a rebellion in which the Parthians took part.

In the troubles which broke out upon the death of the Pseudo-Smerdis, B.C.521, Parthia revolted, in conjunction (as it would seem) with Hyrcania, espousing the cause of that Median pretender, who, declaring himself a descendant of the old Median monarchs, set himself up as a rival to Darius.

Hytaspes, the father of Darius, held at this time the Parthian satrapy.