The article draws on previously unknown documents from the Russian State Archive of Ancient Acts to study structure and size of the Ukrainian army under command of I. Zolotarenko, which participated in the Russo-Polish war of 16541655. This war with the Polish-Lithuanian Commonwealth was an important event in the history of Central and Eastern Europe, and the first success of a joint Ukrainian-Russian military action in the early modern period. Contrary to the prevailing opinion of the historiography, registers on pay distribution preserved in the ‘Little Russia affairs' fond allow to assert that I. Zolotarenko's army numbered 18,000 in enlisted ranks (not counting starshinas); it was divided in 6 regiments and included in addition to registered Cossacks, volunteer Cossacks, hajduks, and German mercenaries. Apropos, this refutes the prevailing assertion that the Zaporozhian Host enlisted no foreign mercenaries. The author also concludes that there was a good reason for Zolotarenko to call himself ‘Severian Hetman. ' He created a kind of ‘superstructure' over the regiments participating in the campaign, which included army (but not regiment) officers and his own ‘court.' The documents in question give a sense of how the Ukrainian Hetmanate organized its military campaigns under the direction of specially appointed hetmans. Article also refutes the opinion of modern Ukrainian historians that while administering the oath of regiments in February 1654, the tsar's representatives mechanically transferred whole paragraphs from Cossack registers. In fact, oath books' structure was fundamentally different from that of Cossack registers. The article also expounds the changes in administrative structure of the hetmanship occurred in 1654. Since the register of I. Zolotarenko's troops is the third list of registered Cossacks known to historians, it is obviously worth publishing. Moreover, the troops' structure of the Ukrainian hetmanate dovetailed its administrative-territorial division, which underlay all its executive and judicial structures.