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.