About half a million people have downloaded a mobile phone ringtone featuring the phrase "Por qué no te callas?" or "Why don't you shut up?" leading Madrid daily El Pais reported on its Web site.
That's what King Juan Carlos told Venezuelan President Hugo Chavez during a heated confrontation at a summit in Chile last week.
The ringtone is thought to have generated around €1.5 million (US$2.2 million) for the companies selling it, El Pais said.
Some companies offer ring tones with the king's actual voice, while others use actors, apparently out of copyright concerns.
T-shirts and mugs featuring the words are also becoming a profitable business, and videos of the confrontation have been a hit on the YouTube Web site.
Chavez's opponents in Venezuela are no less obsessed.
Pirated copies of the quote have been popping up in the South American country.
In Venezuela, T-shirts with the slogan in Spanish have the "NO" in uppercase - a call for voting against constitutional reforms that would significantly expand Chavez's power. The Venezuelan leader says the changes would empower neighborhood-based assemblies and advance the country's transition to socialism.
"The king said what Venezuelans have wanted to say to Chavez's face for a long time," said Jenny Romero, 21, a student sporting one of the T-shirts in Caracas. "I'm wearing this T-shirt to protest everything bad that has happened in the country."
The spat last week began when Chavez repeatedly called former Spanish Prime Minister Jose Maria Aznar a "fascist."
Spain's current prime minister, Jose Luis Rodriguez Zapatero, asked Chavez to be more diplomatic and show respect for other leaders. As Chavez repeatedly tried to interrupt, King Juan Carlos leaned forward and said: "Why don't you shut up?"