A method, has been developed for selective fragmentation of T7 DNA at AT-rich regions. The molecules have been subjected to complete digestion with single-strand-specific SI endonuolease after fixation of DNA AT-rich regions in the denatured state by glyoxal. The treatment resulted in three fragments having molecular weights of 13.6±0.4, 8.2±0.4 and 3.5±0.16 megadaltons as determined by electron microscopy. The position of these fragments along the T7 DNA molecule has been determined by means of analysis of the intermediates during SI-cleavage.