Darius Henderson grabbed the winner as Millwall came from behind to beat Peterborough.
The home side took the lead when Paul Taylor and Lee Frecklington linked to find George Boyd inside the area and he drove in a low shot.
But Millwall were level before the break when an Alan Dunne cross found Scott Malone, who made no mistake.
And Henderson won it when he latched on to Andy Keogh's pass to shoot low past Robert Olejnik.
Darius Henderson hit 19 goals for Millwall last season
Darren Ferguson's side have now lost both of their league games and find themselves in 23rd place.
Boyd had missed from close range early on but made amends when he turned 12 yards out and fired a left-footed shot into the bottom right-hand corner.
From there on in Millwall were on top with Keogh, Josh Wright and Liam Trotter all going close before Malone's equaliser.
Dunne broke clear down the right and when the ball was deflected into the path of Malone, he blasted a shot beyond Olejnik from the left side of the six-yard box.
Olejnik produced a good parrying save to deny Henderson at the start of the second half and James Henry saw a shot hit a post before Henderson hit the winner.
Keogh held up a long ball forward and laid it off to his strike partner, who struck home from 12 yards out.
The margin of victory was almost increased with 10 minutes remaining but Henry hit the woodwork once more, this time striking the crossbar from long range.
Peterborough manager Darren Ferguson told BBC Radio Cambridgeshire:
"We started off so well, George gave us the lead and you'd think we'd be on the front foot then.
"But it seemed to have the opposite effect and I have to say Millwall deserved the win.
"It's never a good time to lose a goal (just before half-time), but I was going to make changes anyway because I felt we needed more energy in the team and a little bit more tempo.
"It didn't have the desired effect and in most departments they were better than us."
Peterborough United 01 Olejnik 03 Alcock 04 Brisley 05 Zakuani 24 Ntlhe 06 Bostwick 17 Newell (McCann - 46' ) 23 Frecklington (Swanson - 73' ) 08 Tomlin 10 Boyd Booked 26 Taylor (Sinclair - 46' ) Substitutes 13 Day 19 Little 07 Swanson 11 McCann 20 Knight-Percival 12 Sinclair 22 Clarke-Harris Millwall 33 M Taylor 02 Dunne 12 Lowry Booked 27 Osborne 28 Malone 04 Wright 06 Trotter 14 Henry (Batt - 85' ) 26 Abdou 07 Henderson 20 Keogh Booked Substitutes 01 Forde 03 Shittu 08 Racon 15 Feeney 19 C Taylor 09 Marquis 11 Batt