We describe the mechanical separation of the two complementary strands of a single molecule of bacteriophage λ DNA. The 3' and 5' extremities on one end of the molecule are pulled progressively apart, and this leads to the opening of the double helix. The typical forces along the opening are in the range of 10-15 pN. The separation force signal is shown to be related to the local GC vs. AT content along the molecule. Variations of this content on a typical scale of 100-500 bases are presently detected.