Morgan Whittaker scored the only goal against former club Swansea to give Plymouth their first away victory of the season.
The striker – who left the Swans in a £1million move in July last year – scored in the 18th minute to give the Pilgrims a 1-0 win.
It was Whittaker’s 17th goal of a profitable season and his ninth in his last 10 games as Argyle made it six games without defeat.
Back-to-back Championship defeats and a fourth loss in a row in all competitions leaves Swansea head coach Luke Williams still searching for his first league win since he took charge.
It also leaves the Swans just five points clear of the relegation zone.
Liam Cullen struck the bar late on, but Swansea lacked accuracy in front of goal and missed several good chances.
Both clubs had been active on transfer deadline day and their respected line- ups reflected their work.
Swansea duo Przemyslaw Placheta and Charles Sagoe Jr were among their substitutes, while Ronald – who joined the club earlier in the week – was given his first start.
For Plymouth, Lino Sousa – like Sagoe Jr, an Arsenal loanee – was named on the bench.
After Swansea’s Ben Cabango had spurned an early chance by lashing a shot high and wide from the edge of the six-yard box, Whittaker gave early indications of his threat.
The deep-lying striker’s opening gambit was thwarted when Nathan Wood blocked a shot, before Whittaker tested Carl Rushworth with a stinging volley the Swansea keeper just about managed to tip over.
From the corner, Plymouth fashioned a goal. Adam Forshaw started the move and then found Whittaker with a pass which the striker clipped nonchalantly, first-time, low into the corner from the edge of the box.
Swansea had already felt an injustice when a penalty appeal for a push on Ollie Cooper was turned down, but their failure to level before half-time lay with themselves.
New signing Ronald – a £1.5m buy from Brazilian club Gremio Anapolis – twice set up his team-mates from the right wing, but neither chance was taken.
Firstly, Jamie Paterson skied his shot over the bar. Then, Jerry Yates allowed Plymouth keeper Conor Hazard to save after the Swans striker met another accurate Ronald cross into the goalmouth.
It was the same story at the start of the second half as Yates mis-directed his header wide of the near post from the Brazilian’s delivery.
If Argyle were riding their luck a little, they were also still looking dangerous enough to improve their lead on the counter-attack.
Just before the hour mark, Mickel Miller fired over the bar after two team-mates had shots blocked in a frantic few seconds.
Swans’ substitute Placheta looked lively after coming on for his debut, but when the Polish winger created space for himself on the edge of the box, his effort was well clear of the bar.
Placheta then crossed for fellow substitute Cullen, but the striker was unable to beat Hazard from close range.
Swansea were now building a head of steam as Plymouth tired and came nearest to an equaliser when Cullen’s dipping shot struck the top of the crossbar in the 84th minute.
Hazard also saved from substitute Charlie Patino in the third minute of added time.