Scott Brown celebrated a first away win in charge of Fleetwood as a 1-0 triumph extended Peterborough's slump.
Brown saw captain Josh Vela hit the only goal midway through the first half as they snapped a run of five successive draws in League One and condemned Posh to a fifth consecutive defeat in all competitions.
It was a case of third time lucky for the Cod Army as Vela classily volleyed past defender Nathan Thompson on the goal-line in the 23rd minute after Carlos Mendes Gomes was twice denied by Posh keeper Lucas Bergstrom.
Vela's strike was the first to be conceded by Posh on their own turf in League One so far this season and was enough to end their previously perfect record of home wins.
A game of few chances may well have turned out differently had Jack Marriott not blasted over the bar a few minutes before the visitors hit the front.
Marriott also fluffed another good opportunity in first-half stoppage time but Posh rarely looked like levelling in the second period, when captain Jonson Clarke-Harris forced Town keeper Jay Lynch into his only save of note.