As fairy godmothers go, Betty Brown Casey is pretty nice to have on your team.

The low-key philanthropist has a passion for the Washington Opera, a passion that has translated into millions in donations and support for the organization. Last night Casey threw a gala concert for 2,500 fellow lovers of the opera -- and picked up the entire tab herself.

"We're here to say thank you to all of you for all the years in the Washington Opera family," she told the audience. Casey ticked off a list of the thankees: volunteers, staff, board members, subscribers and "those of you who sat -- year after year -- quietly, patiently and resignedly, in seats next to those who loved opera -- and you didn't." Long-suffering husbands in tuxedos broke into huge grins as knowing laughter rippled through DAR Constitution Hall.

The program included mezzo-soprano Denyce Graves, soprano Veronica Villarroel, bass Rene Pape, conductor Valery Gergiev and the Three Mo' Tenors. (Artistic Director Placido Domingo was scheduled to conduct and sing at the gala, but was sidelined by stomach flu.)

"Tonight is wonderful," said Betty Vertiz, a Washington Opera subscriber since the 1960s. "We even like our seats!"

Three generations of her family attended the gala: husband Oscar Vertiz, his daughter Virginia Cameron and granddaughter Carrie Gouskos. "It's nice for people who are faithful to the opera to feel they're appreciated," she said.

All because Casey wanted to do "something nice" after the risky move to Constitution Hall, the Washington Opera's temporary home this year while the Kennedy Center Opera House undergoes renovations. The cost of last night's soiree? "It's a private party," she demurred, but a savvy eye would chalk up seven figures.

"She's been the absolute soul of the company," Domingo said last week. "She always wants to do more and thank anybody who's been involved with the company in any capacity."

Casey, sitting nearby, flushed with embarrassment. The philanthropist shrinks from anything that smacks of self-promotion, and agreed to speak to a reporter only to highlight the contributions of everybody else.

"It's just that this company went through some hard times and there were many, many people who worked very hard to not only keep us going and to make us better and better over the years. I just felt it was a good time to say thank you to Placido -- who has been the real spark plug for everything that has happened to us -- and to everybody. We really feel like a family, so I felt we should have a family reunion."

Casey, 75, has had a soft spot for opera since she was a teenager. "I just love the music," she said with a smile. "I get into the music and I'm just there. Terrible as it may seem, Placido, there are times when I don't care who's singing. I just love the music."

Luckily for the opera, Casey is in a position to nurture that love. After 31 years of marriage to legendary Maryland developer Eugene Bernard Casey, she inherited an estate of more than $200 million when he died in 1986. She has led a very private life since then, quietly doling out donations to her pet projects.

"I just think that everybody in life does what they can do," she said. "I'm naturally shy, and I'm just more comfortable when people don't think I do anything -- because I don't feel like I do. I only do things that I really believe in, I only do things that I can afford, and I don't do things I ask other people to give to. I don't start something and then ask other people to give me money to do that project. So I don't try to bother anybody, so to speak."

"Betty knows, and some of us, we know it," said Domingo. "And that's enough."

Her support is funneled through the Eugene B. Casey Foundation to the Salvation Army, Suburban Hospital, George Washington University and Georgetown University and its hospital. She generated more than a few headlines when she offered to build an official residence for the District's mayor on a 17-acre estate in Northwest Washington, and created a $50 million endowment to plant and tend the city's trees.

Casey has a special affection for the Washington Opera. She joined the board in 1974 and has been a member ever since; she now holds the title of life chairman. In 1996, she spent $18 million to buy the Woodward & Lothrop building with the idea of converting it into a state-of-the-art opera house in the heart of downtown Washington. When the opera decided to remain at the Kennedy Center instead, the company was allowed to sell the building and keep the profits.

"She's terrific," said Opera President Michael Sonnenreich. "She's stepped up and exhibited a leadership role for the opera beyond financial. She's setting examples for others to follow."

Last night's gala cames after the company's successful move to Constitution Hall -- an artistic experiment that, so far, has generated praise from critics and subscribers alike.

The evening began with a standing ovation for Casey, who thanked everyone who had contributed to the success of the 47-year-old company. She asked for whistles, bravos and bravas for two individuals who had carried the opera during the tough times: former general director Martin Feinstein and longtime board member Christine Hunter.

The program was full of familiar material -- and a few surprises. The strongest applause came for Pape, who sang two arias for his Washington debut: "Le veau d'or" from Gounod's "Faust," and "Ella giammai m'amo" from Verdi's "Don Carlo." Tenor John Matz had his role unexpectedly expanded when he filled in for Domingo in "Granada." Washington native Graves had the widest repertoire, with a French aria and an American spiritual, and the Three Mo' Tenors were also all over the map with the classic "La donna e mobile" followed by "Let the Good Times Roll."

The good times kept rolling after the concert, when 300 guests joined Casey for dinner and dancing at the Organization of American States across the street. The grand ballroom was lavishly decorated with spring bouquets, Peter Duchin kept the dance floor hopping, and the speeches were short but sincere. "I am so impressed by Betty," said Washington Opera Chairman Jim Kimsey. "Without her the opera would not be what it is today."

The hostess was characteristically low-key about the evening. "I thought it was wonderful, really great," she said. "Perfect, really, except for Placido" not being here.

Washington Post music critic emeritus Joseph McLellan contributed to this report.

Philanthropist Betty Brown Casey picked up the tab for last night's Washington Opera gala.Three Mo' Tenors -- Rodrick Dixon, left, Thomas Young and Victor Trent Cook -- give it their best at Constitution Hall.Russian soprano Tatiana Pavlovskaya performs at a Constitution Hall concert for 2,500 Washington Opera patrons -- paid for entirely by philanthopist Betty Brown Casey, above with opera President Michael Sonnenreich.