Arianna Forster, 48, who gained acclaim in the late 1970s as Ari Up, the singer of the exuberant and influential all-female punk-rock band the Slits, died Wednesday in Los Angeles.

Her death was reported on the website 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 Forster died after "a serious illness."

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, when they released their first record, "Cut," whose cover art showed three shirtless young women, including Forster, they had forged a unique sound. The album included songs with titles like "Shoplifting." In the late '70s the Slits toured as the opening act for the Clash, then the reigning protagonists of political punk rebellion.

She said her inspiration came in 1975, when her mother brought her to a Sex Pistols concert and she saw 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 said, 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.

Vivien Goldman, a journalist and a friend of Forster, said of her 1970s persona: "You cannot be a female artist on the wild side, very passionate and self-expressive, without being formed at least in part by Ari. In her feral 14-year-old way, she did represent a new archetype of womanhood."

NEW YORK TIMES