Skipper Ed Joyce fell on the stroke of tea to give Somerset the advantage in their relegation battle with Sussex at Hove.
Joyce had looked in imperious form until he was strangled down the leg side by Jamie Overton having made 83.
It left Sussex in a spot of bother on 162-5 at tea with Mike Yardy unbeaten on 12 in his final match at Hove before retirement.
Sussex had resumed on 72-2 after lunch but lost Chris Nash for 11 when he edged Peter Trego to Marcus Trescothick at second slip.
Luke Wright added 48 with Joyce for the fourth wicket before he fell for 20, Trescothick again pouching the catch at second slip off the bowling of Jim Allenby.
Joyce brought up his third half century of the season by cutting Tim Groenewald for four and looked a good bet to reach a hundred until he tried to flick Jamie Overton off his hips and was caught behind.
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