BEFORE THE Beatles, before Jimi Hendrix, there were the Isley Brothers. Before soft rock and disco and rap, before Prince and Living Colour and the Black Rock Coalition . . . And they're still very much with us. In fact, they'll be at Constitution Hall on Saturday.

The Brothers got their start in the business at age 13, and since then they've left their fingerprints all over rock history. They were raised in the gospel tradition, and their first hit "Shout, Parts 1 & 2" in 1959, was really a gospel "ring shout" with guitar, bass and drums plugged in (it was a reworking of their improvised concert finale). Another early hit, "Twist & Shout," released in 1962, jumped aboard the Twist craze, and made the U.S. Top 20, but got its biggest boost when the Beatles recorded it in 1963.

In 1965, the Isleys hired a young backup guitarist named Jimi Hendrix, and watched his name and fame eclipse their own two years later. And then there's the Isley's four-decade chain of hits, stretching from "Shout" to the more recent "Smooth Sailin' Tonight," a chain which winds through nearly every subgenre of pop. And lead singer Ronald Isley says the ever-adaptable, always evolving Isley Brothers will be ready when the next new thing comes along.

At 44, Isley looks entirely too young to be a founding member of one of the founding groups of rock and R&B. "Well, we work hard at that," he laughs. "And we started kind of young -- I was 13 when it all began."

With family friend and co-producer (and Ronald's significant other) Angela Winbush, the Isleys have been on tour since March, and on a rare day off, Isley's calling from his Los Angeles office. Brothers Marvin and Ernie also live in California, and Rudolph lives in Oklahoma.

The three oldest Isley boys -- Ronald, Rudolph and the late O'Kelly -- made a reputation for themselves as gospel singers in their hometown Cincinnati, performing with their mom, Sallye Bernice Isley. After watching the secular success of such gospel-rooted singers as Ray Charles and Jackie Wilson, the Isleys got permission to go to New York to sell their kind of soul. They worked the Apollo Theatre in Harlem and the Howard Theatre in Washington, and developed a reputation as a blistering live act, going out with Motown's Motortown Revue for a time.

When the next wave of Isleys was old enough, the Isley Brothers added younger brothers Ernie on lead guitar and Marvin on bass, and cousin Chris Jasper on keyboards, and became a self-contained vocal and instrumental unit (O'Kelly died in 1985). The infusion of young blood expanded on the traditional soul of "This Old Heart of Mine" and the uncut funk of "It's Your Thing," and moved into the spacy psychedelic guitar rock of "That Lady," the distinctively Isley-ized versions of '70s soft rock hits by the likes of Seals & Crofts and James Taylor, the black power anthems "Fight The Power" and "The Pride," and boudoir ballads like "For the Love of You" and "Who Loves You Better," and even disco with "(It's a) Disco Night."

"At the very beginning, our parents taught us to do it all, from gospel to you name it," Isley says. "So flexibility has always been important to us -- keeping an eye on the signs of the times."

Somewhere among all those hits, after years of jumping from label to label, and manager to manager, the Isleys finally took matters into their own hands and started their own record label, a virtually unprecedented business move for any group, let alone a black group. They called it T-Neck, named after Teaneck, New Jersey. "It was kind of hard at the beginning," Isley says. "But we got an education by being out there with Motown, seeing how it was done."

The Isleys have often been mentioned as strong black rock influences by Living Colour, and distaff rappers Salt 'n' Pepa scored hits with funked-up versions of the Isleys' "It's Your Thing" and "Twist and Shout." But along the way the influencers have been influenced themselves -- their recent albums feature the most up-to-date electrotechnology, and the "Smooth Sailin' " album featured a rap by Kool Moe Dee. Isley says rap doesn't play much of a part in the current tour, "although I talk a lot," he laughs.

"Our show is better now than it's been since I don't know when," he says. "I guess it's the combination of all those songs -- we're doing everything. We dig way back, from 'Shout' all the way up to 'Spend the Night.' "

The band is billed as the Isley Brothers Featuring Ronald Isley, but Isley quickly points out that he's not star-tripping -- it's a matter of clarification. "We're just letting people know it's the same lead singer that did everything. At one point, part of the group -- the younger brothers Ernie, Marvin and Chris went off and did another couple of albums as a spinoff band. Now they're back with us, but a few people got confused by that. This is the real thing."

So what explains the Isley Brothers' longevity?

"It's the love of the music -- it's our hobby, our everything," Isley says. "And being family doesn't hurt either. We can't get away from each other."

THE ISLEY BROTHERS Featuring Ronald Isley -- Appearing Saturday with Angela Winbush at Constitution Hall. Call 800/543-3041.