The Welsh county's chances of recording their first Championship win of the season diminished despite winning the toss and electing to bat, with the First Division's bottom club finding partnerships difficult to establish.
David Hemp struck 31 but the captain, Robert Croft, was the only other batsman to surpass the 30-run mark, the veteran's unbeaten 66 being crucial to Glamorgan's first-innings total of 239.
Dougie Brown was the pick of the bowlers with 4 for 58, and the South Africa paceman Makhaya Ntini took the final wicket of the innings.
The deficit was reduced after an unbeaten half-century from the opener Ian Westwood, although the captain, Nick Knight, chipped in with 36. The Bears had reached 165 by the end of play.
The Middlesex opener Ed Smith struck a fine century at Trent Bridge to halt the progress of title-chasing Nottinghamshire. Wickets fell regularly, but Smith and James Dalrymple managed a fourth-wicket partnership of 139, the latter contributing 60.
Smith was eventually bowled by the former England all-rounder Mark Ealham for 128 and the visitors were dismissed for 325 on the first day after winning the toss.
Nottinghamshire were 33 without loss in reply as they looked to take advantage of their game in hand on Kent and Hampshire above them.
Sussex were bowled out for 365 after winning the toss against Gloucestershire in Cheltenham. Carl Hopkinson, Murray Goodwin, Chris Adams and Matt Prior all hit half-centuries, but no player made a three-figure total, partly because of Malinga Bandara's four-wicket haul.
Gloucestershire lost Kadeer Ali early on in their reply, caught behind off Naved-ul-Hasan as the hosts reached 28 for 1 at stumps.
In the Second Division, Phil Jaques and Michael Lumb hit centuries against Somerset at Taunton as Yorkshire maintained their promotion push. The pair shared a 119-run stand as the hosts toiled after losing the toss. Jaques eventually compiled 106 while Lumb struck 130.
The former England bowler Andrew Caddick was the pick of the bowlers with 3 for 87 as Yorkshire reached 377 for 7 at stumps.
Derbyshire, bottom of the Second Division, gave themselves a chance of their first victory of the season by bowling out Northamptonshire at Wantage Road. The hosts were dismissed for 140, with the opening bowler Ian Hunter taking 4 for 50. Michael DiVenuto (43) and Stephen Stubbings (44) got off to a good start as the visitors reached 135 for 4 after the departure of their openers.
There was a notable milestone for Andrew Caddick when he claimed his 1,000th first-class victim. The Yorkshire opener Joe Sayers edged the third ball of the match to James Hildreth at second slip to guide Caddick to the landmark.Reuse content