The highest mountain of the East Antarctica Ranges, near the coast of Queen Maud Land. Large mountain rising to 3,425 m, standing 4 mi SE of Mount Bergersen in the Sor Rondane Mountains. Mapped by Norwegian cartographers in 1957 from air photos taken by U.S. Navy, 1946–47, and named for Major Gunnar Isachsen, leader with Captain Hjalmar Riiser-Larsen of the Norwegian expedition 1930–31.