Glamorgan paceman David Harrison put the skids under Worcestershire by taking a career-best seven for 45 in the LV= County Championship Division Two match at New Road.
The 28-year-old was in impressive form to dismiss the hosts for 134 in 47.5 overs with Moeen Ali making a defiant 85 not out.
By the close Glamorgan were 228 for four off 46 overs after being boosted by Ben Wright's unbeaten 71 which contained one six and eight fours off 99 balls.
The eventful opening day, however, belonged to Harrison who proved far too hot to handle for Worcestershire whose only bright spot in a gloomy innings was Moeen's innings which included 13 fours off 135 balls.
It followed the 126 and 70 not out he scored in his side's previous Championship match when they drew with Surrey at Whitgift School in Croydon.
New Road skipper Vikram Solanki's decision to bat after winning the toss was soon under the microscope after half his side were out for 62 in 17 overs on an occasionally unpredictable wicket.
Their alarming decline began when Phil Jaques departed to the fourth ball of the day when he pushed forward to James Harris and was caught behind by Mark Wallace for his third Championship duck on the trot.
The Australian's fellow opener Daryl Mitchell departed shortly afterwards when he got a leading edge to a Harrison delivery and was snapped up at mid-off by Huw Waters.
The procession continued when Solanki fended Harrison to Jamie Dalrymple at second slip while Alexei Kervezee nicked Harris to Wallace.
Ben Smith, the only other Worcestershire batsman to reach double figures by making 15, played down the wrong line to Jim Allenby's third delivery and was trapped lbw before in-form Moeen completed his well deserved half-century with nine fours off 66 balls.
Ben Scott was caught low down at cover by Gareth Rees off Harrison immediately before lunch when Worcestershire's score stood at a grim 88 for six off 28.4 overs.
Their plight soon began to worsen after the interval with Richard Jones, Alan Richardson and Jack Shantry all being trapped leg before by Harrison.
Six-footer Harrison then picked up his fourth wicket after lunch, for a cost of just 18 runs, when he had Imran Arif splendidly caught in the slips by Mark Cosgrove
Glamorgan did not have it all their own way in the early stages of their reply with Cosgrove being caught in the deep by Shantry when going for a big hit off Arif while Mike Powell was bowled by Richardson after surviving a slip chance before getting off the mark.
Rees and Wright steadied matters, however, with a 59-run stand in 12 overs before Rees (42) fell to Shantry who then removed Dalrymple. By then Glamorgan had already taken a first innings lead.