England star Kevin Pietersen struck a majestic century after James Tomlinson had ripped through Somerset to leave Hampshire in charge on the opening day.
Seamer Tomlinson took advantage of some poor shot selection on a greenish pitch to claim a career-best 8-46.
The home side were shot out for 126 and although Hants lost both openers cheaply, Pietersen was soon on top.
He hit 14 fours in making 100 before edging behind, while John Crawley was on 78 as the visitors closed on 194-3.
Injury-hit left-arm seamer Tomlinson went into the game with only 49 first-class wickets in six years.
But he took the first six Somerset wickets at a cost of just 12 runs in 4.2 overs to leave the Cidermen in tatters at 23-6.
In his second over he struck with his second, third and fifth deliveries to bowl Neil Edwards, have Justin Langer caught at backward point and James Hildreth snapped up at short-leg.
Marcus Trescothick and Zander de Bruyn provided slip catches and Peter Trego was lbw first ball as Tomlinson swung and seamed the ball.
There was also a maiden Championship wicket for 18-year-old seamer Hamza Riazuddin, who bowled tidily on debut.
But Tomlinson was by no means unplayable and Craig Kieswetter (32) and Ben Phillips (39) showed what to do in adding 50 for the eighth wicket before Tomlinson removed the latter's off-stump with a ball which held its line.
Phillips was last man out - having hit seven fours before being caught at slip - to complete Tomlinson's haul.
The wickets continued to tumble as Hampshire openers Michael Brown and Jimmy Adams fell to Charl Willoughby and Phillips.
But Pietersen, making a rare Championship appearance, was a figure of authority a week before the first Test against New Zealand.
He reached his half-century off 58 balls but having reached 100 off 158 he was out to the next delivery, caught behind off Trego attempting a back-foot forcing shot.
Crawley was untroubled after a watchful start and reached his fifty off 106 deliveries, with six fours.
~RS~q~RS~~RS~z~RS~58~RS~)