Pace bowler James Fuller became only the fourth Gloucestershire player to take a hat-trick at Cheltenham as Worcestershire were bowled out for 182 on the opening day of the LV= County Championship match.
Having won the toss, Worcestershire crashed to 22-6, Fuller removing Matthew Pardoe with the final ball of the eighth over and then striking with the first and second deliveries of the 10th, sending back Moeen Ali and Alexei Kervezee.
Joe Leach came to Worcestershire's rescue with a career-best 82 not out, sharing a seventh-wicket stand of 120 with Gareth Andrew, who made 54. Fuller finished with 5-43 from 19 overs.
By the close, Gloucestershire had made a confident start to their reply and were 83 without loss, Chris Dent unbeaten on 56 and Michael Klinger 27 not out.
The first Gloucestershire player to take a hat-trick at the College Ground, where the club have been playing fixtures since 1872, was Charles Townsend back in 1893. The feat was repeated by Mike Procter in 1979 and James Franklin in 2009.
Fuller joined the group after having Pardoe and Kervezee caught behind, with the wicket of Moeen, who played on trying to withdraw the bat, sandwiched in between.
In the over that divided Fuller's three wickets, Will Gidman had Daryl Mitchell taken by wicketkeeper Gareth Roderick and from 18 without loss Worcestershire nosedived to 18-4.
Worse was to follow in Craig Miles' first over as the tall seamer had both Thilan Sameraweera and Ben Cox caught at second slip by Dent off successive balls.
It was left to Andrew and Leach to steady a sinking ship before Dent and Klinger put the home side in total command before stumps.