Single linear DNA molecules were bound at multiple sites at one extremity to a treated glass cover slip and at the other to a magnetic bead. The DNA was therefore torsionally constrained. A magnetic field was used to rotate the beads and thus to coil and pull the DNA. The stretching force was determined by analysis of the Brownian fluctuations of the bead. Here, the elastic behavior of individual λ DNA molecules over-and underwound by up to 500 turns was studied. A sharp transition was discovered from a low to a high extension state at a force of ~0.45 piconewtons for underwound molecules and at a force of ~3 piconewtons for overwound ones. These transitions, probably reflecting the formation of alternative structure in stretched coiled DNA molecules, might be relevant for DNA transcription and replication.