Mexico's ruling National Action Party (PAN) has become the largest party in Congress for the first time.
The results of the presidential election are under dispute
But the final results of the 2 July elections, released on Wednesday, saw the party fall short of the outright majority required to govern alone.
The PRD came in second place, ahead of the Institutional Revolutionary Party which governed for more than 70 years.
The PAN and the PRD are involved in a dispute over the outcome of the vote for president held at the same time.
The left-wing candidate, Andres Manuel Lopez Obrador, has challenged the initial result, which found that conservative Felipe Calderon - of the PAN - had won by a margin of less than one percentage point.
The Federal Electoral Tribunal has until the end of August to rule on the legal challenges and until 6 September to name a president-elect.
In the legislative elections, the PAN won 206 seats in the 500-seat Chamber of Deputies and 52 seats in the Senate.
Mr Obrador's Party of the Democratic Revolution (PRD) won 126 seats in the lower chamber and 29 in the Senate.