During the War of the Fourth Coalition, Marulaz led a cavalry brigade at the Battle of Golymin on 26 December 1806. At the Battle of Eylau on 8 February 1807, he led Marshal Louis Davout's III Corps cavalry brigade. Under his orders were the 1st and 12th Chasseurs à Cheval Regiments, the 2nd Chasseurs having been assigned to other duties. At first his troopers covered the advance of Louis Friant's division, then drove off some cossacks. Marulaz took a position guarding the right flank and repelled an attack by Russian cavalry. He became embroiled in the bitter fight over possession of the village of Klein Sausgarten. During Davout's subsequent advance, Marulaz protected the extreme right flank of the III Corps. Late in the afternoon, his cavalry were attacked by the late-arriving Prussians near the village of Kutschitten and driven back along with the rest of Davout's troops. The 12th Chasseurs of his brigade was present at the Battle of Heilsberg on 10 June 1807.