#INSERT type:image caption:Monterrey's Edgar Castillo is one of 154 players in Liga MX who were not born in Mexico. END#
More than 30 percent of players in the Mexican League for Apertura 2015 were not born in Mexico, according to figures released by the league this week.
The number of foreign players remains unchanged over last season, according to Mexico daily, La Aficion.
Of the tournament's 475 players, 154 were not born in Mexico. The list includes 81 foreigners, 55 naturalized citizens and 18 players who have dual citizenship. Six teams have 10 or more players who were not born in Mexico: Xolos, Querétaro, Cruz Azul, Atlas, America and Tigres.
Tijuana began the trend of dual-citizenship players by registering players born of Mexican parents as Mexico nationals. The club has the most non-Mexico born players in the top division.
Along with the five league-allowed foreign born players, Tijuana has four more naturalized players and another four players born in the United States whose parents are Mexican.
Cruz Azul and Querétaro each have 12 non-Mexican born players - the five league-allowed foreigners plus six naturalized citizens each and one dual citizenship player apiece.
For the Apertura 2014 tournament, Liga MX rules were changed to allow naturalized Mexicans to play as domestic players as soon as they gained citizenship; previously, they had to complete five years in the country to count. The catch is that South Americans can obtain Mexican citizenship and count after just two years in the country.
Clubs encourage players to become Mexican when the two years are up; it opens up one of the five foreign-player spots each Liga MX club is permitted, giving them more options as to whom they can sign. The result has been the number of naturalized players swelling to over 50 for this present Apertura 2015.
Over the past weekend, America, Queretaro and Chiapas all started their matches with seven foreign-born players. And in the top four squads in the league (Tigres, Club America, Monterrey, Toluca), a total of just three Mexicans under the age of 25 began games.
The Liga MX sets the regulations, but it is the clubs and coaches who decide which players to field each week. They have tended toward filling foreigner spots, picking experience over youth, and naturalizing the foreigners as quickly as possible.
ESPN FC's Tom Marshall contributed to this report.