Sussex then reached the close on 70 for 2 with Ian Ward on 42. Ward added 51 for the second wicket with Mike Yardy, who was caught at short midwicket off the spinner Dean Cosker.
Richard Montgomerie fell in the fourth over, pinned in front by Alex Wharf. Glamorgan, who won the toss, subsided after being 144 for 2 at lunch. David Hemp top-scored with 71 and added 86 for the third wicket with Jonathan Hughes (32). The captain, Robert Croft, scored 35 to bring up 10,000 first-class runs in his career.
Glamorgan made a poor start when Dan Cherry got an inside edge and was bowled. Mushtaq struck with the first ball of his second over with Ryan Watkins caught at mid-off, before bowling Hughes after lunch, and Kirtley removed Mark Wallace and Hemp, his 500th first-class wicket for Sussex. The slide continued as Mushtaq had Wharf taken at silly point, and Michael Powell was later pinned in front by Yardy.
But Croftdominated an eighth-wicket stand of 40 with Cosker, who became Mushtaq's fourth victim. Croft was trapped lbw by Kirtley, and Mushtaq ended the innings by bowling David Harrison.
A century from Justin Kemp rescued title-chasing Kent on the opening day of their game at Lord's with Middlesex. Kent won the toss but were 249 for 9 when Kemp was joined by the last man Amjad Khan. The pair put on 135 for the final wicket as the visitors were bowled out for 384.
In the Second Division, Yorkshire had a first-innings lead in their sights after bowling out Worcestershire at Headingley. Three wickets for Tim Bresnan helped bowl the visitors out for 308.
Anthony McGrath hit an unbeaten half century with the hosts 96 for 2, 212 runs behind, at stumps.
Somerset's captain, Ian Blackwell, fell two runs short of a century against Northamptonshire at Taunton. The hosts reached 382 for 7 after winning the toss.