Paul Horton's only two previous tons this season were both in the FP Trophy
Paul Horton's first County Championship century of the season held Lancashire's batting together to improve their chances of avoiding relegation.
The opener hit a career-best 155 not out as Lancashire made 297-5 against Somerset at Taunton after being invited to bat first under cloudless skies.
Kyle Hogg contributed an unbeaten 69, helping to dig the visitors out of trouble at 153-5.
Their unbroken partnership of 144 changed the complexion of the day.
Ben Phillips (3-43) and Charl Willoughby (2-88) had earlier put Somerset on top. But Lancashire, who started the day 19 points clear of trouble, were in a more comfortable position by the close.
The hosts were without skipper Justin Langer because of a shoulder injury sustained while fielding in the Pro40 match against Essex on Tuesday.
Marcus Trescothick took over, as he is expected to do for next season, and duly won the toss.
Opening bowlers Willoughby and Alfonso Thomas did not enjoy much luck early on as former Somerset player Luke Sutton and Horton built an opening stand of 58.
It was broken when stand-in opener Sutton, on 26, needed treatment for a blow on the head from a short ball from Phillips.
The next delivery saw him edge a leg-side catch to wicketkeeper Craig Kieswetter.
It was 89-1 at lunch with Horton one short of his half-century. But Somerset hit back strongly in the afternoon session.
Willoughby started the fightback by pinning Mal Loye lbw for 14, then VVS Laxman was caught behind for 12 chasing a wide ball from the same bowler. And Mark Chilton went for a duck, also caught behind off Phillips.
Francois du Plessis did not last long either, driving at a wide one from Phillips to give Kieswetter his fourth catch, having made only a single. And Lancashire, at 153-5 in the 50th over, looked in disarray.
But Horton was still there, reaching a gritty hundred with a pulled four off David Stiff.
Somerset could have had him on 121, only for Kieswetter to spill a diving chance off Zander de Bruyn.
By then Hogg was well set, the tall left-hander moving confidently to his 50 off 85 deliveries.
But there was no stopping Horton, who ended the day by surpassing his career-best score of 152, which he has twice reached, first against Hampshire at Old Trafford in 2007 and again last summer against Yorkshire at Headingley.
Lancashire batsman Paul Horton told BBC Radio Lancashire:
"They had a period where they bowled well and managed to nick a few wickets to keep them in the game.
"We weren't surprised they won the toss and bowled, we would have done the same.
"We still have less than 300, so I've only done half a job. We need to force the victory to make sure we don't get relegated."
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