He replaced Hauptmann Heinrich Sannemann who was transferred and led the Staffel until late April when Zimmermann took command.

He succeeded Leutnant Dieter Zink who had been shot down by anti-aircraft artillery and taken prisoner of war on 11 July.

[4] On 18 December during the Battle of the Bulge, Zimmermann claimed his 29th aerial victory when he shot down a Republic P-47 Thunderbolt fighter.

[7] According to Obermaier, Zimmermann was credited with 34 victories in over 580 combat missions, 28 of which claimed on the Western Front including 14 heavy bombers.

Of this figure, he claimed six aerial victories on the Eastern Front and more than 22 over the Western Allies, including eleven four-engined heavy bombers.