IN a tastefully luxurious suite at the Peninsula Hotel in New York, Brian Johnson, the lead singer of AC/DC, is crooning the opening lines of “It’s a Wonderful World” while he waits to have his picture taken. Although he’s known for belting out AC/DC’s hard rock songs, he can also sing delicately about trees of green and red roses.

Suddenly he clears his nasal passages with a giant snort and cracks up laughing. “What’s green and goes backward at 100 miles an hour?” he asks in a northeastern English accent. Across the room the brothers Angus and Malcolm Young, the band’s guitarists, start laughing along with him. Even at their age  Angus is 53, Malcolm is 55 and Mr. Johnson is 61  the members of AC/DC can’t resist a gross-out joke.

The band’s music hasn’t matured much either, to the delight of its fans. AC/DC has always delivered an aggressive take on rock’s raw essentials: slicing guitars, driving rhythms and lyrics about sex, drink and rock ’n’ roll. Its new album, “Black Ice” (Columbia), which will be sold in the United States only at Wal-Mart starting Oct. 20, is its most focused release in almost two decades, full of the fist-pumping riffs and shout-along choruses the band is known for. And it is expected to be one of fall’s biggest rock releases.

Gradually, and without getting much media attention, AC/DC has become the most popular currently active rock band in the country, to judge by albums sold. Since 1991, when Nielsen SoundScan started tracking music sales, this Australian band has sold 26.4 million albums, second only to the Beatles in catalog sales, and more than the Rolling Stones or Led Zeppelin. Over the past five years, as CD sales have cratered, AC/DC albums have sold just as well as or better than ever; the band sold more than 1.3 million CDs in the United States last year, even though it hasn’t put out any new music since 2000. And with “Black Ice,” increased visibility for the band’s catalog at Wal-Mart and a tour that starts Oct. 28, it’s possible that AC/DC could sell more CDs overall this year than any other act in pop music.