Arturo Beltran Leyva, shot dead during a shootout with security forces, was one of Mexico's most wanted men, with a $2.3m reward on his head.
Known as "the boss of bosses", he headed the cartel that bears his name, the Beltran Leyva Organisation (BLO).
This was formed as a gang in its own right after a 2008 split from the notorious and powerful Sinaloa cartel headed by Joaquin Shorty Guzman.
The area of operations of the two gangs along Mexico's Pacific coast overlap to some extent and the two gangs are fighting for control of lucrative smuggling routes into the US market.
In the fluctuating and violent alliances between Mexico's drug gangs, the Beltran Leyva cartel has teamed up with Los Zetas in their deadly feud with the Sinaloa cartel.
Los Zetas are a group of former soldiers hired by the Gulf Cartel as hitmen but now a gang in its own right.
The Beltran Leyva Organisation has been around for a long time and has perhaps the most sophisticated intelligence of any of the gangs, according to a recent report by Stratfor Global Intelligence.
The gang has penetrated every level of Mexican government, Stratfor says.
As well as having a Mexican price on his head, Arturo Beltran Leyva, believed to be around 50, was wanted in the US.
He was formally designated a "drug kingpin". In August 2009, the US justice department indicted him and 42 others in connection with drug-trafficking
And in December 2009, the US Treasury froze assets of people and companies linked to the Beltran Leyva gang.
"The Beltran Leyva Organisation is responsible for acts of terrible violence in the pursuit of money," a treasury statement said, adding that the cartel and its associates controlled firms involved in, among others, transportation, electronics, health products and hospitality.
Washington has accused the Beltran Leyva gang of smuggling millions of dollars of cocaine and heroin into the US.
Their suppliers are Colombian drug gangs, in particular the Norte del Valle gang.
Arturo Beltran Leyva's death is the biggest blow against his cartel since January 2008 when one of his brothers was captured.
However, the leadership gap is likely to soon be filled. Three other brothers, including Hector Beltran Leyva, are accused of involvement in the cartel.