Somerset 142 for 5 (Abell 52*) lead Hampshire 79 (Gregory 4-26) by 63 runs
A fighting half-century from skipper Tom Abell helped Somerset build a 63-run lead over Hampshire on an eventful opening day at the Ageas Bowl that saw 15 wickets fall.
Abell was unbeaten on 52 - guiding his side to 142 for 5 - after Hampshire were bowled out for just 79 having been put in to bat on a spicy pitch in chilly and cloudy conditions.
Lewis Gregory was the pick of the Somerset attack, taking 4 for 26, as the hosts were dismissed inside 41 overs less than an hour after lunch.
It was the first time since 1969 Hampshire had posted back-to-back sub-100 first-innings totals following last week's 92 all out against Surrey at The Kia Oval.
The home side started the day in third place in Group Two - one spot behind Somerset - and did well to only lose the in-form Ian Holland inside the opening hour. Holland departed for four when Gregory found the outside edge and Craig Overton took the catch at slip.
But the second hour of the day proved to be a disaster for Hampshire as five wickets fell for 16 runs inside 11 overs with Tom Alsop, Sam Northeast, James Vince and Liam Dawson all posting single-figure scores.
Opener Joe Weatherley top-scored for his side with a scratchy 20 but was guilty of running out Northeast for just four after he took a risky single and George Bartlett hit the stumps with a direct hit from cover.
Gregory mopped up the tail 50 minutes after the lunch break having received excellent support from the admirable Overton and Josh Davey, who each picked up two wickets.
The overhead conditions continued to favour the seamers in the afternoon session and Somerset fell to 11 for 2 inside four overs when new-ball pairing Keith Barker and Mohammad Abass dismissed Tom Lammonby and Eddie Byrom cheaply.
Kyle Abbott then trapped James Hildreth lbw for 14 before Abell and Bartlett posted the first 50 partnership of the match following a short rain delay. The 54-run stand was broken by Abbott tempting Bartlett, who had played well for his 27, into playing at a rising delivery and Dawson snared a high catch at second slip.
Abell and Lewis Goldsworthy held firm against some top-class bowling until the penultimate over of the day when Holland beat the defences of Goldsworthy with a full-length ball that clattered into the stumps.
But Holland then went from hero to villain when he grassed a regulation catch off nightwatchman Jack Leach who had edged Barker to third slip.