Ernst Beyeler, one of the world’s leading postwar art dealers and collectors whose career stretched six decades and included the co-founding of the Art Basel art fair and a renowned museum that bears his name, has died of natural causes at his home in Riehen, Switzerland, according to officials at his Fondation Beyeler. He was 88.

The son of a Swiss railroad worker, Beyeler’s career in the art market began in the 1940s, when he worked with antiquarian bookseller Oskar Schloss in Basel, Switzerland, while attending university. Following Schloss’s death in 1945, Beyeler took over the operation of the shop, transforming it into one of Europe’s most formidable galleries by showing the work of artists such as Monet, Matisse, Klee, Giacometti, and Picasso. The Fondation Beyeler estimates that the dealer sold more than 16,000 works over the course of 60 years, and held more than 300 exhibitions in his gallery.

In 1982, Beyeler and his wife, Hildy, transferred their estimable collection of art — believed to be worth as much as $1.85 billion — to a private foundation for preservation and study, though the two continued to operate their gallery together. First shown publicly at Madrid’s Reina Sofia in 1989, the works in their collection eventually found a home in a building designed by Pritzker Prize-winning architect Renzo Piano, which opened in Basel in 1997.

Beyeler served as president of his foundation until 2009, when he resigned. Hansjörg Wyss currently serves as president of the foundation’s council; Georg Krayer as the administration’s council; and Sam Keller, a former Art Basel director, as the director of the museum.