Karen Stupples, Gloria Park and Chiharu Yamaguchi share second place at 4-under-par 67.
Defending champion Annika Sorenstam posted a 3-under-par 68 and is tied for fifth with Moira Dunn, Stephanie Louden and Grace Park. Park lost to Sorenstam in a playoff last year, but claimed the 2004 season's first major, the Kraft Nabisco Championship.
Sorenstam started on the back nine Thursday and flew out with two birdies in her first three holes. She added a birdie at No. 18 to make the turn at 3 under, which was one shot out of the lead.
The No. 1 player in the world dropped shots at 3 and 7 to fall down the leaderboard. Sorenstam birdied her final two holes to get within two of the lead.
Grace Park took a different path to her spot on the leaderboard. She was 5 under through 14 holes, thanks to six birdies and a lone bogey. Problems arose for the season's first major winner at No. 17.
She hit a punch 6-iron right of the hole and made bogey. The runner-up last year also bogeyed 18 to fall into the group in fifth.
'This is not the finish I wanted to have,' said Park. 'I'm certainly disappointed and frustrated all at the same time, but that's golf. That's major championships.'
As eyes were on Sorenstam and Park on Thursday, it was Park's youth rival that stole the show.
Rosales, who played junior golf and collegiately against Park, parred her first three holes on Thursday, but broke into red figures for the first time at the par-4 fourth. She knocked a wedge to 10 feet and rolled home the birdie putt.
She parred her next four and closed the front nine with a 6-footer for birdie at the par-5 ninth.
Rosales used strong wedge play after the turn to make her move up the leaderboard. She hit a 50-degree wedge to 10 feet to set up birdie at the 11th, then hit the same club 5 feet farther at No. 12 for her fourth birdie of the round.
The 25-year-old reached the green with her second shot at the par-5 16th and two-putted for birdie from 35 feet. That birdie gave Rosales the first-round lead for the first time on the LPGA Tour.
'My round was pretty smooth today,' said Rosales. 'I hit a lot of fairways today and that was the key to making birdies. I missed a lot of putts out there, but I kept putting and putting until I made some.'
Rosales won for the first time on tour this season at the Chick-fil-A Charity Championship and that win might be the key to her first successful run in a major championship.
'I'm happy the way I played, but we'll just take one day at a time and see where I end up on the weekend,' she said.
Stupples mixed five birdies over her first 16 holes, including a chip-in at seven, and appeared set to keep first for herself. At the 18th, she hit a 6-iron left of the green then hit a poor chip. After another chip, Stupples tapped in for par and a share of second.
'I really hit the ball well today, sometimes too well,' said Stupples, who won the season-opening Welch's/Fry's Championship. 'I hit over a couple of greens when I really struck it well.'
Gloria Park was at 4 under as she played her 16th, No. 7 at DuPont. She missed the green with a 9-iron and failed to get up-and-down for par at the hole. She rebounded with an 8-footer for birdie at her last to get into second.
'I would like to win a major, but this is only the first day, and I don't really think about winning the majors at this time,' admitted Gloria Park. 'I want to enjoy out there every day, and one hole at a time.'
Yamaguchi was flawless on Thursday with four birdies and no bogies.
'I played a very good round I think,' said Yamaguchi, a tour rookie. 'Yeah, it's very close to perfect, I think. So I'm pretty happy to do it.'
Former champions Se Ri Pak and Meg Mallon joined Kim Saiki, Seol-An Jeon, Soo Young Moon, Angela Stanford, Sherri Steinhauer, Soo-Yun Kang, Cristie Kerr, Pat Hurst, Shi Hyun Ahn and Carin Koch in a tie for eighth place at minus-2.