Molecular combing is a powerful and simple method for aligning DNA molecules onto a surface. Using this technique combined with fluorescence microscopy, we observed that the length of X-DNA molecules was extended to about 1.6 times their contour length (unextended length, 16.2 mum) by the combing method on hydrophobic polymethylmetacrylate coated surfaces. The effects of sodium and magnesium ions and pH of the DNA solution were investigated. Interestingly, we observed force-induced melting of single DNA molecules. (C) 2004 American Institute of Physics.