Her death was reported on the Web site of her stepfather, John Lydon, also known as Johnny Rotten, the singer of the Sex Pistols and Public Image Ltd. The site (johnlydon.com) said that Ms. Forster died after “a serious illness.”
Ms. Forster, who was born in Germany, was 14 years old when the Slits were founded in 1976 and, like many of their punk-rock contemporaries, picked up instruments with an energetic do-it-yourself ethos but little formal training.
By 1979, however, when they released their first record, “Cut,” whose cover art showed three shirtless young women, including Ms. Forster, wearing loincloths and daubed with mud, they had forged a unique sound out of their self-taught approach. The album included songs with titles like “Shoplifting.”
“They had embraced rhythms beyond basic punk, while their lead singer, Ari Up, reveled in every quaver and yelp,” the critic Jon Pareles wrote in The New York Times in 2009. “She sang mocking critiques of trendiness, romance and consumerism as the music juggled rock, ska, reggae and something like funk. The Slits came up with odd-angled chord progressions that better trained musicians wouldn’t touch.”
In the late ’70s the Slits toured as the opening act for the Clash, then the reigning protagonists of political punk rebellion. The Clash’s singer and guitarist, Joe Strummer, taught chords to Ms. Forster; the other Clash guitarist, Mick Jones, helped her band mates tune their instruments.
Some might have been intimidated by proximity to such high-profile musicians, but while growing up in London Ms. Forster became used to hanging out with luminaries. In an interview with the music Web site Pitchfork, she described living with her mother, Nora Forster, in a house where Jimi Hendrix was a regular visitor and said that Jon Anderson, the singer of the psychedelic English band Yes, was her godfather.
In 1975 her mother brought her to a Sex Pistols concert, where Ms. Forster was struck by the sight of a frenetic Johnny Rotten — her mother’s future husband — hurtling across the stage. The next year she went to see the Clash, and it was there, she told Pitchfork, that she met a drummer called Palmolive and together they founded the Slits. The first song the two played together was “Blitzkrieg Bop” by the Ramones.
After their second album, in 1981, the Slits broke up. But more than 20 years later, Ms. Forster and a Slits bassist, Tessa Pollitt, restarted the band. In 2009 the Slits released a full-length album, “Trapped Animal,” and on Thursday Ms. Forster’s label, Narnack Records, released a video for a song from that record, “Lazy Slam.”
Between the two incarnations of the Slits, Ms. Forster lived in Belize, Indonesia, New York and Jamaica, where, using the name Madussa, she immersed herself in dancehall reggae.
She is survived by her mother and three sons, Pablo, Pedro and Wilton.
Vivien Goldman, a journalist and a friend of Ms. Forster, said that her 1970s persona, characterized by an unapologetic attitude and combination of feminine and utilitarian garb, was an inspiration for women who followed her into the world of punk rock.
“You cannot be a female artist on the wild side, very passionate and self-expressive, without being formed at least in part by Ari,” Ms. Goldman said. “In her feral 14-year-old way, she did represent a new archetype of womanhood.”
