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, the visitors crashed to 22 for six, Fuller removing Matthew Pardoe with the final ball of the eighth over and then striking with the first and second deliveries of the 10th to send 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 4-43 from 19 overs.
The home side then made a confident start to their reply and were 83 without loss at stumps, 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 scalps, Will Gidman had Daryl Mitchell taken by wicketkeeper Gareth Roderick, meaning the first four Worcestershire wickets fell with the score on 18.
Worse was to follow in Craig Miles' opening 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 with determination and application, but both needed lives early on.
They took the total to 63 for six at lunch. Andrew subsequently moved to his half-century off 91 balls, with 10 fours, and Leach soon followed suit, his fifty also featuring 10 fours and occupying 109 deliveries.
For a while Gloucestershire bowled without much luck. Then skipper Klinger pulled off a stunning catch above his head at short cover off Benny Howell to dismiss Andrew.
Shaaiq Choudhry soon followed, caught behind to give Fuller his fourth wicket, and at tea Worcestershire were 154 for eight.
The final session saw Graeme Cessford dismissed for a duck on his championship debut by Tom Smith, before Fuller wrapped up the innings by having Alan Richardson caught in the slips.
The ball seamed around early in the day and also swung, yet conditions looked very different when the hosts replied.
Klinger and Dent, who reached 50 from as many deliveries, produced positive shots from the outset and looked in little trouble.