Glamorgan's bowlers turned the screw on Worcestershire after Glamorgan secured maximum batting points off the last ball of the 110th over before eventually being bowled out for 449.
Wicket-keeper Tom Cullen struck a fourth half-century in seven games and was aided by tail-end cameos from Lukas Carey, Timm van der Gugten and Michael Hogan.
In reply, Worcestershire reached 191/5 at the close, still 258 runs behind with Hogan leading the attack with 2/28 in 16 overs.
"I'd rather be us than them at half-way, it was a pretty good day for us considering we bowled for two sessions with the sun out and good batting conditions, I'll take my hat off to the bowlers," said Cullen.
"Michael Hogan's Mr Consistent. he's been very good for a very long time and continues to be the leader of our attack.
"Another 50 for me, I'd like to contribute more but to get that last batting point makes a difference. Chris Cooke's due to come back next game (from injury), but if I can put as much pressure as I can to get in as a batter, then it's good for the club."