In search of her first LPGA Tour win, Castrale had the only bogey-free round of the day to take a one-stroke edge over Brandie Burton, Angela Stanford and former Oklahoma State star Karin Sjodin.
Castrale got a favorable kick off a ridge at the 13th hole, then made a 4-foot birdie putt to get to 4 under. After hitting into a greenside bunker at No. 17, she maintained her lead with a par-saving 15-footer.
The 28-year-old former Southern California player had 20 players within three strokes of her on the leaderboard, including top-ranked Lorena Ochoa, Kraft Nabisco champion Morgan Pressel, Paula Creamer and Se Ri Pak.
'You really can't think that you have the lead,' Castrale said. 'You just have to execute shots. I can play well and not win or I can play well and win, so really all I can do is control myself.'
The best finish of Castrale's career was a third at the Honda LPGA Thailand in October. Before that, she finished fourth in the Fields Open last February, but then missed the cut in four of her next six events. She hasn't missed another cut since, and added another fourth-place finish to her resume last month at the Ginn Open.
'I just try to keep it real simple. I got off to a pretty good start this year, and I just figured if I keep doing what I'm doing, good things will happen,' Castrale said.
Castrale chipped in for birdie after an errant 8-iron shot at No. 3, then followed it with a 25-foot birdie putt at the fourth hole. She added another birdie at No. 7 before making the turn.
Aree Song was the only other player who reached 4 under. She had two bogeys in her final five holes to finish at 69 with Pak, 2001 winner Gloria Park and four others.
Ochoa, who took over the No. 1 ranking last week, fought through cramping in her left hand to finish with a 70, three strokes behind Castrale. Ochoa, Pressel and Creamer were among a group of 10 golfers tied for 12th.
Ochoa bogeyed No. 3, but came back with three birdies between the 11th and 15th holes.
'I knew I needed to make some birdies and put myself in a good position, so I was a little more aggressive on the back nine,' Ochoa said. 'I made a few putts that put me in a good rhythm. I'm going to try to do the same tomorrow on my front nine, and hopefully I will make a lot of birdies.'
Ochoa had a couple chances for birdies early on in the first round, but couldn't get her putts to fall on the par-5 fifth or par-3 sixth before tapping in for pars on both.
'I wanted to do better, but I think being in the red numbers and being close to the leaders is a good position,' the Mexican star said. 'Tomorrow's going to be important. Tomorrow is moving day. Tomorrow is Cinco de Mayo, so maybe for my Mexicans I will play low.'
Ochoa said she didn't think the cramping in her hand was anything severe.
'I'm going to go and check that, but hopefully I'll be good for tomorrow,' she said.
After two days of soaking rain, an already challenging course at Cedar Ridge Country Club was playing even longer than its 6,602-yard measurements -- even though players teed off in front of soggy tee boxes at three holes and players were allowed to wipe mud off golf balls that landed in the fairway.
'Well, we kind of had to play lift, clean and place. That's how wet it is out there. There wasn't really much of a choice,' Pressel said. 'There was mud on just about every shaft and they haven't been able to cut the fairways because they can't get the mowers out. That would destroy the golf course.'
Defending champion Cristie Kerr, who set the course record with a 61 last year, birdied two of the first three holes and remained at 2 under through 12 holes before a double bogey at No. 13 set off a collapse. She had three more bogeys to end up at 74 and refused to talk to reporters after the round.