Prugh became the last of eight players to shoot 6-under 66 at Shadow Hills Country Club, a group that combined to make 53 birdies and just five bogeys to share a one-shot lead.
Todd Fischer, Daniel Summerhays, Matthew Jones and Mike Heinen all got around the course with a bogey. Jason Day, Roland Thatcher, Kelly Grunewald and Prugh joined them in the lead.
It matched the Nationwide Tour record for most players tied for the 18-hole lead, a mark originally set at the 2001 Ozarks Open.
They held a one-shot advantage on Henrik Bjornstad and Brad Elder, who were at 5-under 67. Seven players were tied at 68 and nearly half the field broke par.
It all happened in fall conditions.
'I'd take a 6-under round on this golf course any day, especially when it is this cold and damp,' said Thatcher, the tour's money leader, who played in 50- degree weather in the morning.
'I'm looking forward to coming out here tomorrow and maybe getting some warmer weather. I was home in Houston last week in 110 heat index, so today was a little different.'
Thatcher finished off his 66 at the same time as Day, who became the youngest winner ever in a PGA TOUR-sanctioned event at this year's Legend Financial Group Classic.
'I'm normally a pretty slow starter. I play better on the weekends,' said Day, 19. 'I've got myself into good position early, so that should help with three days left to play.'
Prugh, a University of Washington standout, is tied for the lead following his first round on the Nationwide Tour -- and his first 18 holes in competition as a professional.
Co-leader Summerhays knows a little something about being in that position. He won the Children's Hospital Invitational as an amateur this season, then turned professional.
'I feel like turning professional and getting out here has me so much more prepared for next season,' said Summerhays. 'Having the exemption and a guaranteed place to play next year is much more important than the money I could have made if I had turned professional before winning.'
Among the co-leaders, only Thatcher, Day, Fischer and Heinen have either held or shared a first-round lead before on the Nationwide Tour.