Pedro Troglio's Gimnasia La Plata side made it five straight wins with a 2-1 victory at home to bottom side All Boys.
A goal difference of +6 puts them narrowly on top, ahead of River Plate (also 27 points, +5), Estudiantes (26, +6), Colon (26, +4) and Godoy Cruz (25, +5).
Lucas Licht and Gustavo Bou were on the scoresheet for the league leaders at the Estadio Juan Carmelo Zerillo on Monday.
It took just eight minutes for the hosts to take the lead and it came in style.
Javier Mendoza crossed from the left and Licht's stunning overhead kick from inside the area was perfectly placed into the bottom corner.
Bou doubled the advantage on 69 minutes and while relegation-threatened All Boys netted through Jonathan Calleri, they were unable to find a second.
Omar Pouso was sent off for the hosts in the 88th minute after picking up a second yellow card.
River Plate are level on points with the leaders after Teofilo Gutierrez's first-half goal saw them past Velez Sarsfield 1-0.
A dreadful goal-kick provided Gutierrez with the chance and he made no mistake in the one-on-one.
Estudiantes slipped to third after being held to a 0-0 draw at Rosario Central and Colon were kept to a goalless draw at home to Belgrano.
Godoy Cruz claimed the biggest win of matchday 15, cruising past Atletico Rafaela 3-0.
A Juan Eluchans own goal in the first half sent the hosts on their way before Julio Rodriguez and Diego Rodriguez added efforts after the break.
San Lorenzo are winless in three matches after playing out a second consecutive 0-0 draw, this time against Newell's Old Boys.
Relegation-threatened Argentinos Juniors suffered a 1-0 loss at home to Racing Club, who had Rodrigo de Paul score a dramatic late winner.
Olimpo edged Quilmes 1-0, Arsenal struck three second-half goals in a 3-1 win over Lanus and Juan Roman Riquelme's stunning late 30-yard effort saw Boca Juniors past Tigre 1-0.
