After Operation Barbarossa, the German invasion of the Soviet Union, began on 22 June 1941, Yermolkin fought on the Southwestern Front with his unit. As part of the 26th Army, the division and its corps began the war in the area of Drogobych. During the Border Battles on 25 June 1941 as part of the corps the division launched a counterattack in the area of Brody and Dubno, and then in fierce fighting retreated towards Proskurov and Vinnytsia. Then-Major Yermolkin was wounded by shrapnel during fighting in the area of Radzivilov on 28 June. In August during the Battle of Kiev he was appointed commander of the 300th Motor Rifle Regiment of the 7th Motorized Division, leading it in fierce fighting south of Kiev as part of the 26th Army. Yermolkin was caught in encirclement in September but managed to return to Soviet lines in October, after which he received command of the 680th Rifle Regiment of the 169th Rifle Division, engaged in heavy fighting as part of the 38th Army of the Southwestern Front in the Volchansk region. From December he commanded the 584th Rifle Regiment of the 199th Rifle Division of the army. In July 1942 Yermolkin became deputy commander of the 304th Rifle Division of the army during the Donbass Defensive operation.