The Prince of Wales and Camilla attended a performance of the musical Beach Blanket Babylon, in San Francisco. The spoof show was especially altered for their US visit.
The prince met "the King" after he and his wife, the Duchess of Cornwall, joined the audience in a standing ovation.
The heir to the throne told performers - who included fellow royal "King Louis" -: "I think you all need a stiff drink after this."
The show's producer, Jo Schuman Silver, said special tributes to the couple were added, but they were not targets of the show. "It's just something we wouldn't do," she said.
Beach Blanket Babylon is the longest running hit musical in the US. The Queen attended a performance of the show in 1983.
Charles and Camilla have received little criticism in the media since arriving in California. Arriving at church on Sunday, the Prince greeted journalists: "You're still here."
The prince and duchess sung hymns and said prayers at the Episcopalian St Columba's parish church, in Inverness, near San Francisco.
The daughter of church pastor Tom Brindley gave the duchess a bouquet.
One member of Pastor Brindley's congregation said: "It's just like any other service, but with a special guest."
Prince Charles and Camilla will visit an "edible schoolyard" - where pupils grow and cook their own food - on Monday, before the prince speaks at an environmental seminar.