LaLiga champs Barcelona suffered a defeat on the opening day of the season after a superb winner from veteran Aritz Aduriz handed Athletic Bilbao victory.
Barca produced a lacklustre performance and were punished when substitute Aduriz, 38, found the net with a stunning 89th minute scissor-kick.
Luis Suarez and Rafinha both hit the woodwork for Barca but the visitors struggled to impress despite fielding new £107m signing Antoine Griezmann.
Suarez was forced off with an injury.
Former Spain forward Aduriz has already announced he is retiring at the end of the season after a career spanning 20 years.
He threw himself into the air and acrobatically volleyed past Marc-Andre ter Stegen to earn Athletic Bilbao a first league win over Barcelona since 2013.
