Somerset 314 (Barrow 88, Compton 79, Willey 4-64) and 235 for 6 (Myburgh 87, Willey 3-47) drew with Northamptonshire 448 for 9 dec (Duckett 144*, Middlebrook 61, Thomas 4-116)
Johann Myburgh celebrated the signing of a two-year contract by guiding Somerset to a draw against Northamptonshire on the final day at Taunton which confirmed the visitor's relegation.
Myburgh, the 33-year-old South African, whose new agreement was announced on Tuesday, made 87 as the hosts closed on 235 for 6 in their second innings, leading by 101 runs as David Willey took 3 for 47.
Marcus Trescothick and Tom Abell helped frustrate a Northants attack delayed from taking the second new ball by poor light in the final session.
By the time it was made available Somerset were 196 for 4, 62 runs ahead with only 23 more overs possible and about to condemn their opponents to the mathematical certainty of dropping back to Division Two.
David Ripley, the Northamptonshire coach, said: "While it's disappointing to be relegated with such a small points total, I think we can genuinely point to a fair bit of misfortune this season.
"We had three overseas players lined up and for one reason or another none of them joined us. Add to that a catalogue of injuries and it has been difficult. We have had good sessions, but not enough good matches. This was as well as we have played and I felt we were in control of the game for a lot of the time.
"Ben Duckett played a fantastic innings and if we can hang onto our good young players we can build for another crack at first fivision cricket.''
Northamptonshire took 12 points from the game, while Somerset's 10 ended their mathematical chance of lifting the title.
They began the day on 38 without loss, trailing by 96. Myburgh, who has enjoyed a consistent first season at Taunton, and Trescothick took the total to 74 before the latter fell lbw on the back foot to Willey.
It looked a situation tailor-made for Nick Compton but he had made only 7 when Andrew Hall beat his forward defensive push and knocked back the off stump. Abell, who also signed a new contract this seek, helped move the score to 98 for 2 at lunch, which was taken with Myburgh on 36.
The pair continued to play solidly in the afternoon session, Myburgh reaching his half-century off 155 balls with a six over long on off James Middlebrook.
Abell followed up his 95 on debut in the previous Championship game against Warwickshire with another mature innings for a 20-year-old before losing his wicket with a lapse in concentration.
Myburgh played a shot to short fine leg and was called for a quick single by his partner, who was run out by Willey's direct hit at the wicketkeeper's end.
Tea was taken at 175 for 3 with a draw looking certain. But James Hildreth, having survived a chance to midwicket in the first over after the break, was bowled by occasional offspinner Rob Keogh for nine attempting a forced shot off the back foot.
Keogh was only bowling because of the light. When it brightened Northamptonshire took the second new ball for the 86th over and Willey immediately struck twice with it.
Myburgh's patient 255-ball innings - featuring 10 fours and a six - ended when he was pinned lbw by a ball that appeared to keep low and two balls later Peter Trego, who had edged his first delivery to third-man for four, was bowled.
At 200 for 6, Somerset led by 66 with a further 22 overs possible and Northamptonshire sniffed an opportunity to claim their first Championship win of the summer.
But Alex Barrow and Craig Overton snuffed it out, the latter surviving a slip chance on 8 off Willey as they added 35 before the players shook hands at 5.15pm.