Jamison spoke from Ailey's New York headquarters while her dancers performed in Iowa City - the 16th stop on a nationwide tour celebrating the company's 50th anniversary.

A Philadelphia native, Jamison, 64, vividly recalls seeing the great Cuban ballerina Alicia Alonso perform at the Academy a half-century ago: "My mom took me to see the Ballets Russes when I was 10 years old. We sat in the top row and somehow got backstage afterwards, where I got her autograph. It was an indelible experience."

And Jamison, a Germantown High School grad, became a ballerina, despite the obstacles she faced as a tall (5-foot-10) African American woman in a historically white art form. But her life really changed in 1965, when she joined Ailey's modern-dance company and quickly became a star.

During her 15 years as a performer with the Ailey troupe Jamison captivated audiences, most notably in her spectacular solo, Cry. Then, after a period spent appearing as a guest artist with other companies and headlining the Broadway musical hit Sophisticated Ladies, she came back to Ailey to take over for its ailing director, who died in 1989. For the last two decades Jamison has sustained, and enriched, the troupe in myriad ways.

To fully appreciate her accomplishments you have to consider how unusual it is for any U.S. dance company to last one decade, let alone five. Alvin Ailey is financially healthy (also rare among American dance troupes), with a packed performance schedule. It has appeared in 71 countries on six continents; it boasts a large repertoire of works by Ailey and other distinguished choreographers; and, since 2004, it has been based in its own custom-designed eight-story building on 55th Street at Ninth Avenue in Manhattan. There is a junior company (Ailey II), an Ailey School, summer camps, extension classes open to the public, and much more.

As Jamison notes, all this reflects "a responsibility to the two aspects of 'the Ailey mantra': dance excellence, plus always being aware of the fact that you started somewhere - someone opened that door for you."

The Ailey troupe has long been noted for the unusually strong technique, versatility and physical attractiveness of its company members. But, Jamison emphasizes, "it's not just about dancing in a vacuum. We were 'reaching out' before the buzzword was invented."

That sense of social responsibility to the community is reflected in the Ailey programs that bring the arts into public schools, and in the troupe's commitment to making dance training accessible to talented individuals at all economic levels, from all ethnic backgrounds.

"Mr. Ailey," as Jamison still calls him, made dances "based on our human experience." She clearly delights in the company's diversity: Its 30 members hail from 14 states and the District of Columbia, as well as France. And the Ailey's associate artistic director, Masazumi Chaya, who danced with the company for many years before joining its staff, is a native of southern Japan.

"But we haven't talked about what we'll be doing in Philly!" Jamison said. During the evening programs at the Academy, after a short introductory film (made for the 50th anniversary), Alvin Ailey will launch into Firebird, an updated version of the Russian ballet classic by the flamboyant and controversial French choreographer Maurice Béjart. The company will also present Love Stories, a collaboration involving Jamison, Philadelphia's hip-hop master Rennie Harris, and the radical modern-dance figure Robert Battle. Reflections in D is a pensive, five-minute solo to music by Duke Ellington. Both evenings will end with Revelations, the Ailey's signature work, which remains fresh, and remarkably moving, even as it approaches the half-century mark.

Jamison notes: "When our dancers are first hired they don't feel they're really part of the company till they've performed in Revelations. This piece has withstood the test of time, and is loved by people all over the world."

Danced to traditional gospel songs, Revelations comes from Ailey's "blood memories" of being taken to church as a child, and Jamison - who grew up worshipping at Philadelphia's Mother Bethel A.M.E. Church - clearly understands the fervor behind this work.

The Saturday matinee will feature Ailey's jazzy Night Creature; a work for three men by noted Dutch choreographer Hans von Manen; Twyla Tharp's playful and aerobic The Golden Section, and Revelations.

I asked Jamison about the recent announcement that she will be stepping down as artistic director of Alvin Ailey American Dance Theater in 2011.

"Stepping sideways is more like it," she responded. "Mr. Ailey and I are joined at the hip, and I plan on doing a lot of things" for the troupe. "I'm so excited about the future, and I want the company to be around another 50 years."