Raul Iberbia scored the winner as 10-man Estudiantes beat Argentinos Juniors 2-1 to go to the top of the Clausura, while rivals Tigre and Boca Juniors stumbled in their seventh games of the championship.
Visitors Estudiantes went ahead with a fourth-minute penalty from Mauro Boselli, after a handball by Pablo Barzola, but were pegged back when Nicolas Berardo headed the home side level after 27 minutes.
Iberbia had the decisive say with a close-range finish in the 57th minute after a low centre from Matias Sanchez. Leonardo Jara was dismissed seven minutes later but Estudiantes held on.
Estudiantes climbed above Tigre and Boca Juniors, who both stumbled in their seventh games of the championship.
Tigre were unbeaten with four wins from their opening six games but lost 1-0 away to Arsenal, with Guillermo Burdisso heading the only goal after 65 minutes. Tigre were down to 10 men by then, having lost Angel Diaz to a red card in the 38th minute.
Boca Juniors were held 2-2 at home by Lanus, letting a two-goal lead slip away. They went ahead in the 40th minute with a fluke goal from Pablo Mouche who sent an inswinging cross over from the right and saw the ball sail inside the far post, embarrassing goalkeeper Agustin Marchesin.
Juan Pablo Riquelme doubled the lead two minutes later when his low half-volley found the bottom right corner. But Lanus immediately cut into the two-goal advantage and scored when Mariano Pavone headed down and past Agustin Orion, before levelling in the 55th minute with another fine header, this time from Paolo Goltz.
Boca had Clemente Rodriguez sent off in the 63rd minute but did not lose another goal.
Colon Santa Fe made visitors Independiente suffer, posting a 3-0 win. Ernesto Javier Chevanton opened the scoring after 43 minutes with a composed left-footed finish, before Ivan Moreno y Fabianesi doubled the home side's lead with a deflected strike in the 56th minute.
The ball skidded on the wet pitch and found the bottom right corner. Lucas Mugni snatched the third goal five minutes later, a sweet strike to trump goalkeeper Diego Rodriguez, and Independiente finished with 10 men after substitute Cristian Pellerano was dismissed 11 minutes from the end.