Northampton, missing 10 players to England and the second-string Saxons, were well in touch until the last quarter but were then blitzed by three tries in seven minutes from Olly Woodburn, Stephen Donald and Charlie Beech.
Earlier tries had been scored by Ross Batty and Anthony Perenise - a spectacular sprint by the Samoan prop from nearly 40 metres, while Olly Barkley kicked 21 points.
Saints' points came from a try by prop Alex Waller and three Stephen Myler penalties.
Bath side were missing just four players on England duty. Nick Abendanon, who was not included in the final Saxons squad, started at full-back and hooker Lee Mears was released from the senior camp to sit on the bench. Northampton gave full-back Paul Diggin his first start since October 8, with Myler and Martin Roberts recalled at half-back.
Bath sniffed an opening on the left early on but Abendanon was brought down just short of the line. Saints threatened briefly at the other end before Barkley opened the scoring with a 40-metre penalty in the eighth minute.
Given another more difficult chance, Barkley saw his effort strike the bar. The onrushing Tom Biggs was quickest to the ball but tripped over an outstretched leg. Although referee Sean Davey judged it unintentional he took a harsher view of Myler's marginally high tackle on the winger a couple of minutes later, Barkley making it 6-0.
Myler replied straight away from short range and the two kickers continued to monopolise the scoring as Bath led 12-6 with 27 minutes gone.
Meanwhile, Bath skipper Stuart Hooper, making his return from a broken wrist, lasted just over 20 minutes before taking a knock and being led off, to be replaced in the second row by 20-year-old Will Spencer.
Even with their first-choice props taking a rest on the bench, Saints made life difficult for Bath in the scrum. But Francois Louw was at his burgling best in the loose and played a key role in Batty's try four minutes from half-time. He burst clear on the left before Bath worked the ball right from a ruck and Donald's long pass put the hooker over.
Barkley converted but Myler kicked his third penalty to cut the home lead to 19-9 at the break.
Saints were caught cold straight on the restart as Perenise collected a bouncing ball just inside the visitors 10 metre line and, with a sidestep and a couple of hitch-kicks, touched down under the posts with just 68 seconds gone.
Barkley's conversion stretched the lead to 26-9 but Northampton hit back immediately as prop Waller finished off a catch-and-drive from a line-out. Myler was wide with the conversion attempt.
The third quarter had largely belonged to Saints but they failed to create any clear-cut openings against a well organised home defence. Bath were also showing more intensity and aggression around the tackle and in their driving play than of late and Barkley's fifth penalty edged his side three scores on 64 minutes.
The contest was over when Woodburn took an inside pass to score in the 70th minute but Bath were quickly back on the attack in search of the bonus-point try. Mercer made the telling thrust and appeared in the move again to send Donald over on the left. Barkley could not convert either try but was on target when Beech scored the fifth after 77 minutes.
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