Electrophoretic mobilities of DNA molecules ranging in length from 100 to 10 000 base pairs (bp) were measured in gels of eleven concentrations of agarose from 0.5 to 1.5%. Excellent fits of the dependence of mobility on DNA length were obtained with the relationship 1/mu(L) = 1/mul - (1/mul - 1/mus) e(-L/gamma) showing an e(-L/gamma) crossover, where L is the length of a DNA fragment an gamma is a crossover length ranging from 8000 to 12 000 bp. The other parameters in the fit are V, the mobility of short DNA with unit charge in the limit as length is extrapolated to zero, and mul, the mobility of long DNA as length is extrapolated to infinity. This exponential relationship should be a useful interpolation function for determining DNA lengths over a wide range. The simplicity of this relationship may be of more fundamental significance and suggests that some common feature dominates the electrophoresis of double stranded DNA fragments in agarose gels, regardless of length.