Jason Bohn, who won this year's B.C. Open, fired a 7-under 65 and is tied for third place with Shigeki Maruyama, who shot a 66 on Saturday. The duo is knotted at minus-15.
Trahan was only 1-under par when he reached the par-5 13th. He sank a 12- footer for birdie at that hole, then two-putted from 20 feet for another birdie at the 15th.
The shot that vaulted Trahan into a share of the lead was his approach at 16. From 165 yards, Trahan's 8-iron shot bounced once, then dove into the cup for an eagle.
'It was funny because Justin's (Rose) shot never left the pin before me and we were yelling, 'go in' on his,' said Trahan, a Clemson product. 'Mine came in right behind him. It was amazing.'
Trahan ran over and hugged several members of his family and various friends that came to see him. He composed himself quickly as he knocked his tee ball to 12 feet at the par-3 17th. Trahan's right-to-lefter found the bottom of the cup and the tour rookie found himself at 16 under par.
Trahan found trouble off the tee at 18 when he pulled his drive. He managed to muscle his approach 50 feet short of the stick and his birdie try came up 6 feet shy of the flag. Trahan rolled that in and posted his 16-under-par score.
Choi, playing in the final group on Saturday, was two-under at the turn, but tallied birdies at 12 and 13 to get to 15-under par for the championship. Choi looked to be in trouble at No. 14, but drained a 12-footer to save par.
Choi nearly got up-and-down from left of the green for a birdie at the 15th, but his 20-footer skated over the hole. He ran home a 48-foot birdie putt at the 16th to get to minus-16, but the 35-year-old South Korean had his chances for a solo lead.
At the par-three 17th, Choi hit a four-iron 11 feet short of the stick. His putt died left of the hole, then he missed from 2 feet closer at the last hole, but still has a piece of the 54-hole lead. In his only two PGA Tour wins, Choi had at least a share of the third-round lead.
'I was little nervous on the starting holes,' admitted Choi, whose tour wins both came in 2002. 'The start is important.'
Trahan will be at a disadvantage come Sunday afternoon as his co-leader is 2-0 with the 54-hole lead. Despite some impressive victories as amateur and a few appearances in high-pressure team events, Trahan has never been in this position on the PGA Tour.
'I had some good fortune in my amateur career, I played on some great teams,' said Trahan. 'I was fortunate to be around a lot of great players and play with some good competition. I think it was a great warm-up for me to come out here and get a good career started on tour.'
Robert Gamez, who broke a 15-year winless drought on tour with a win last week at the Texas Open, fired a nine-under 63 and is tied for seventh place with Sergio Garcia (66) and Brandt Jobe (67). They are knotted at 12-under-par 204.
Olin Browne, who captured the Deutsche Bank Championship over Labor Day weekend, posted a 7-under 65 and shares 10th place with Jerry Kelly (67), Mark Wilson (66), Carl Pettersson (69) and International Presidents Cup player Tim Clark (70). The group finished three rounds at minus-11.
John Huston, the second-round leader, struggled to a 2-over-par 74 on Saturday and is part of a group tied for 15th place at 10-under-par 206.