Interaction forces between single strands of DNA were measured with the atomic force microscope by a procedure in which DNA oligonucleotides were covalently attached to a spherical probe and surface. Adhesive forces measured between complementary 20-base strands fell into three distinct distributions centered at 1.52, 1.11, and 0.83 nano-newtons, which are associated with the rupture of the interchain interaction between a single pair of molecules involving 20, 16, and 12 base pairs, respectively. When a third long DNA molecule was coupled between complementary surfaces, both intra- and interchain forces were observed. The intrachain interaction resulting from the molecule’s elasticity manifested itself as a long-range cohesive force.z