Karle, who lost to Paula Creamer in the second round of the 2004 Girls' Junior, started Monday's round with four consecutive birdies. She played bogey-free golf until the 16th hole when she three-putted from 20 feet.
'I woke up this morning and told myself, 'just play a solid round,'' said Karle, whose previous low round was a 67.
Karle rebounded from the bogey to birdie and par the last two holes. She nailed a 5-wood from 72 yards to within a foot on No. 17.
Morgan Pressel, the 2005 U.S. Women's Open co-runner-up, is three strokes behind the leader at BanBury Golf Club.
Sydnee Michaels finished 4 under on Monday and is tied at 67 with Christina Corpus, Juliana Murcia Ortiz and Hsiao-Ching Lu, who was a semifinalist a year ago.
The U.S. Girls' Junior was established in 1949 and is open to any female golfer who has not reached her 18th birthday by the close of the competition and has a USGA Handicap Index that does not exceed 18.4. The Girls' Junior is one of 13 national championships conducted annually by the USGA, 10 of which are strictly for amateurs.