Somerset built up a commanding 538-run lead against Sussex by the end of a second day curtailed by the weather.
James Hildreth resumed on 127 and reached 158 to pass the 4,000-run mark in his first-class career before he was caught at mid-wicket off Ollie Rayner.
Somerset skipper Justin Langer declared on 557 and Sussex suffered an early blow when Chris Nash edged to the Australian without a run on the board.
Michael Yardy and Carl Hopkinson then survived until bad light stopped play.
Earlier, play had not started until 2.30pm because of the weather, with 37 overs being lost as the home side resumed on 399 for three.
Zander de Bruyn passed his 50 but was bowled by Luke Wright as his fourth-wickets partnership with Hildreth came to an end.
Hildreth later followed with his 199-ball innings, which included hit 14 fours and four sixes, finally coming to an end.
Craig Keiswetter and Peter Trego put on a brisk 46-run partnership as the Somerset innings was given a final flourish shortly before Langer decided to declare.
~RS~q~RS~~RS~z~RS~52~RS~)