We have previously shown that asymmetric-voltage field inversion electrophoresis produces more uniform separation for fragments between 1 and 50 kilobases (kb) than other modes of pulsed field gel electrophoresis. We now report on the basis of this phenomenon. As in conventional electrophoresis, the pulsed field mobility of DNAs between 1 and 50 kb varies with voltage In a size dependent manner. The complex migration pattern obtained with asymmetric- voltage field inversion electrophoresis reflects the difference between the mobilities of each sized fragment under the conditions used for the forward and reverse fields. We have applied this technique to DNA sequencing gels and find improvement In resolution for single-stranded fragments in polyacrylamide gels.