Seamer Alfonso Thomas claimed his best figures in English first-class cricket to help Somerset bowl Sussex out for 245 on day one at Taunton.
Ed Joyce (59) and Murray Goodwin (55) laid the groundwork for the visitors after they chose to bat.
But wickets fell at regular intervals before Thomas (6-60) ripped through the middle order to give Somerset the edge.
Off-spinner Ollie Rayner claimed the key wicket of Marcus Trescothick as Somerset closed on 36-1 in reply.
The capture of the Championship's leading run-maker, caught at slip, was a morale boost for Sussex after they had seen 34-year-old all-rounder Thomas bowl with pace and skill in only his third appearance in the competition this summer.
Steve Kirby made the initial breakthrough, having Chris Nash caught at first slip, before Thomas grabbed the first of his six when he had Luke Wells caught behind by wicketkeeper Jos Buttler.
Joyce, who reached his half-century off 92 balls, became Thomas' second victim when he fell lbw in the final over before lunch, and he had both Ben Brown and Rayner caught behind as Sussex reached 215-6 at tea.
Goodwin, after a battling half-century off 107 balls, was caught in the deep off Murali Kartik soon afterwards, leaving Thomas to complete his first six-wicket haul in the Championship by pinning Monty Panesar leg before and having fellow South African Wayne Parnell caught at mid-off.