The self-diffusion coefficient of a series of DNA fragments ranging from 280 to 5386 bases has been measured by fluorescence recovery after photobleaching after thermal denaturation in 8 M urea. The total persistence length p of single-stranded DNAs and its variation in ionic strength down to 10(-3) M has been deduced. The importance of the value of p Versus the pore size a and contour length L of the DNA in the optimization of sequencing by gel electrophoresis is emphasized.