Oxford United missed the chance to return to the top of the Blue Square Premier as Tamworth became the first team to beat them at home this season.
Iyseden Christie scored the game's only goal in the first half, when his cross-cum-shot was fumbled into the net by U's keeper Ryan Clarke.
United ended with 10 men after Damian Batt was sent off for a second caution.
He was booked for dissent after Mark Creighton had a last-gasp effort disallowed for offside.
Oxford were lacklustre and their unbeaten home record came to an end thanks to a brilliant rearguard performance by the Lambs.
Keeper Danny Alcock was in inspired form and frustrated James Constable with two brilliant saves as well as denying Matt Green with his boot in stoppage-time.
Green had another late chance, but he dallied and defender Michael Wylde did well to block his attempt.
The result means that Oxford remain one point behind leaders Stevenage, although they have two games in hand.
Manager Chris Wilder told BBC Oxford:
"I'm massively disappointed and frustrated with the team performance and our inability to make good decisions, to play the right pass and to take our chances.
"We looked like a team who have not played in a while. We were poor today, apart from the first 10 minutes when we should have been 3-0 up.
"I don't know if I'll be facing an FA charge as I was ordered out of his (the referee's) room. I haven't sworn at him, I was just asking questions which is my right. But he wouldn't answer them."
Oxford Utd: Clarke, Chapman (Batt 51), Creighton, Wright, Tonkin, Bulman, Murray, Clist, Potter (Midson 63), Constable, Francis Green (Matthew Green 52). Subs Not Used: Turley, Day.
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