For the second time in his career Nick Selman carried his bat as he finished on 76*, with Glamorgan bowled out on Day 1 for 186 by a fast and accurate Sussex attack.
Sussex finished the day 22 runs ahead as Phil Salt made a century, but he fell to the last ball of the day from Dan Douthwaite to put the game evenly poised heading in to Day 2.
"You have to give Sussex credit for the way they bowled early on," said Selman. "The wicket did a little and it did swing around but we probably lost two wickets too many in the first session.
"The partnership I had with Waggy (Graham Wagg) got us close to 200 and although it hasn't been a great day for us there is still a lot of cricket to play.
"Phil Salt played really well for them and batted with a lot of freedom so it was good to get him in the last over.
"If we can take a couple of early wickets tomorrow and push hard with the ball who knows? We're certainly not out of it."