"Mamma Mia!" made it seem easy to pull off the challenging task of crafting a romantic comedy out of an existing song catalog. Joshua J. Campbell and Suzie Jacobsen Balser co-star in the current production at Mandalay Bay. Bjorn Ulvaeus, left, and Benny Andersson, pose with lead actress Nina Lundseie during the final rehearsal of "Mamma Mia!" in Stockholm, Sweden, on Feb. 2. After having been performed all over the world, the musical made its debut in Ulvaeus and Andersson's native Sweden. THE ASSOCIATED PRESS

So many acts have played Las Vegas over the years, it may not seem like a big deal ABBA was one of them.

But a September 1979 concert at the Aladdin Theater did not merely foreshadow the success "Mamma Mia!" would become all these years later.

It also was one of the few places where American fans got to see the real ABBA. "We only played like, 20 gigs, I guess, in the U.S. ever," says Bjorn Ulvaeus, the quartet's singer, guitarist and co-writer. "You ask Crosby Stills, Nash & Young how many gigs they played," he adds with a laugh.

The group of two couples, at one time both married, had toured Europe for most of the '70s. They had children to consider by then, and realized they were more interested in songwriting than performing live. "We had come to realize, painfully clear at times when we didn't have a hit, that the song is the most important element in this business," Ulvaeus recalls. "So we concentrated on the writing.

"We had also come to realize the recordings themselves, the production, was almost equally important," he adds. So rather than traveling around the world reproducing the stuff we had written before, we decided we could be sitting home writing music."

Ulvaeus hasn't been back to Las Vegas since. Benny Andersson, his songwriting partner, flew in for the opening of "Mamma Mia!" at Mandalay Bay in February 2003.

But Ulvaeus took time for a recent telephone chat from his summer house in Sweden -- a place with a view of "the little cottage where Benny and I wrote most of the stuff 25 or 30 years ago" -- to talk about the "Mamma Mia!" phenomenon, which includes a recent celebration of the 1,000th performance on the Strip.

"Mostly what strikes me is that people have so much fun," Ulvaeus says of the musical for which he carries a producer's credit. "I think that's the amazing thing. I realize they're good tunes and so forth, but 25 or 30 years after -- they're kind of old tunes to me in a way."

"I honestly ask myself, why us?" he adds. "Why those songs, when so many good songs were around in the '70s? And it was so far from our minds sitting there writing them. We thought maybe they would last a year or two."

Producer Judy Craymer noted recently that " 'Mamma Mia!' has become ABBA, in a way. It's kind of funny, because ABBA as a band doesn't exist." Instead, multiple productions of the musical span the world -- "The sun never sets over 'Mamma Mia!' " Ulvaeus jokes -- reaching more audiences than the band ever did.

"It's a good substitute. I think it's better than those bands that go around imitating ABBA," Ulvaeus says.

"Mamma Mia!" is often described as the Broadway hit Ulvaeus and Andersson didn't know they had written. That's somewhat bittersweet after their experience with "Chess," the deliberate attempt at an "Evita"-style pop opera that fell between the ABBA and "Mamma Mia!" years and didn't hit big on Broadway.

"It's the show that (former New York Times theater critic, now political columnist) Frank Rich killed that should have survived," he says of the 1988 effort.

But it made the theatrical stage a familiar place for Ulvaeus, so he was willing to go along with the idea when Craymer and writer Catherine Johnson approached him. "What amazes me is this experiment that we made actually worked out," he says.

"Can you build a story around 23 existing songs and not change the lyrics at all? I took it as a challenge and an experiment. If I felt ever that it would reflect badly on ABBA or the catalog I would have stopped it. But that never happened."

"Mamma Mia!" drew a new audience of theatergoers who may never have been fans of show tunes, but who could still experience the thrill of recognition when a character breaks into a familiar song. But after that fortuitous shortcut, Andersson and Ulvaeus are back to doing it the hard way.

Now they're collaborating with "Les Miserables" lyricist Herbert Kretzmer on an English translation of "Kristina," an ambitious effort about the Swedish emigration to America at the end of the 19th century. The 1995 musical was written in Swedish and "ran here for about three years, which is great for a country that has eight and a half to nine million population," Ulvaeus says.

They hope to cast a workshop production in September to prepare for a Broadway opening in fall of 2006. Ulvaeus acknowledges that while "Mamma Mia!' benefited from a Broadway climate that's obsessed with branding -- hence the trend of adapting movies -- the shoe will now be on the other foot. "It's an incredible risk but one that we will entertain," he says. "We come in with some strength, although with paradoxes that might hurt us. It's miles away from 'Mamma Mia!' We will have to educate those people that if you come to the theater you're in for a completely different experience. That's gonna be a challenge. This is quite serious and very operatic. Benny and I will have to do a lot of promotion," he adds with a chuckle.