Danny Swanson's 29th-minute goal has given Peterborough United the lead against promotion-chasing Watford at half time.
Almen Abdi tested Robert Olejnik in the Peterborough goal just before the break.
Peterborough started the day in the Championship relegation zone, but a win at London Road could see them leapfrog Wolverhampton Wanderers, Barnsley and Huddersfield.
Watford came into the match on the back of a 0-0 draw at home the league leaders Cardiff City.