Japans younger generation came of age with the cellphone, and created its own popular culture by tapping thumbs on keypads.

TOKYO Until recently, cellphone novels composed on phone keypads by young women wielding dexterous thumbs and read by fans on their tiny screens had been dismissed in Japan as a subgenre unworthy of the country that gave the world its first novel, “The Tale of Genji,” a millennium ago. Then last month, the year-end best-seller tally showed that cellphone novels, republished in book form, have not only infiltrated the mainstream but have come to dominate it.

Of last year’s 10 best-selling novels, five were originally cellphone novels, mostly love stories written in the short sentences characteristic of text messaging but containing little of the plotting or character development found in traditional novels. What is more, the top three spots were occupied by first-time cellphone novelists, touching off debates in the news media and blogosphere.

“Will cellphone novels kill ‘the author’?” a famous literary journal, Bungaku-kai, asked on the cover of its January issue. Fans praised the novels as a new literary genre created and consumed by a generation whose reading habits had consisted mostly of manga, or comic books. Critics said the dominance of cellphone novels, with their poor literary quality, would hasten the decline of Japanese literature.

Whatever their literary talents, cellphone novelists are racking up the kind of sales that most more experienced, traditional novelists can only dream of.

One such star, a 21-year-old woman named Rin, wrote “If You” over a six-month stretch during her senior year in high school. While commuting to her part-time job or whenever she found a free moment, she tapped out passages on her cellphone and uploaded them on a popular Web site for would-be authors.

After cellphone readers voted her novel No. 1 in one ranking, her story of the tragic love between two childhood friends was turned into a 142-page hardcover book last year. It sold 400,000 copies and became the No. 5 best-selling novel of 2007, according to a closely watched list by Tohan, a major book distributor.

“My mother didn’t even know that I was writing a novel,” said Rin, who, like many cellphone novelists, goes by only one name. “So at first when I told her, well, I’m coming out with a novel, she was like, what? She didn’t believe it until it came out and appeared in bookstores.”

The cellphone novel was born in 2000 after a home-page-making Web site, Maho no i-rando, realized that many users were writing novels on their blogs; it tinkered with its software to allow users to upload works in progress and readers to comment, creating the serialized cellphone novel. But the number of users uploading novels began booming only two to three years ago, and the number of novels listed on the site reached one million last month, according to Maho no i-rando.

ImageRin, 21, tapped out a novel on her cellphone that sold 400,000 copies in hardcover.Credit...Ko Sasaki for The New York Times

The boom appeared to have been fueled by a development having nothing to do with culture or novels but by cellphone companies’ decision to offer unlimited transmission of packet data, like text-messaging, as part of flat monthly rates. The largest provider, Docomo, began offering this service in mid-2004.

“Their cellphone bills were easily reaching $1,000, so many people experienced what they called ‘packet death,’ and you wouldn’t hear from them for a while,” said Shigeru Matsushima, an editor who oversees the book uploading site at Starts Publishing, a leader in republishing cellphone novels.

The affordability of cellphones coincided with the coming of age of a generation of Japanese for whom cellphones, more than personal computers, had been an integral part of their lives since junior high school. So they read the novels on their cellphones, even though the same Web sites were also accessible by computer. They punched out text messages with their thumbs with blinding speed, and used expressions and emoticons, like smilies and musical notes, whose nuances were lost on anyone over the age of 25.

“It’s not that they had a desire to write and that the cellphone happened to be there,” said Chiaki Ishihara, an expert in Japanese literature at Waseda University who has studied cellphone novels. “Instead, in the course of exchanging e-mail, this tool called the cellphone instilled in them a desire to write.”

Indeed, many cellphone novelists had never written fiction before, and many of their readers had never read novels before, according to publishers.

The writers are not paid for their work online, no many how many millions of times it is viewed. The payoff, if any, comes when the novels are reproduced and sold as traditional books. Readers have free access to the Web sites that carry the novels, or pay at most $1 to $2 a month, but the sites make most of their money from advertising.

Critics say the novels owe a lot to a genre devoured by the young: comic books. In cellphone novels, characters tend to be undeveloped and descriptions thin, while paragraphs are often fragments and consist of dialogue.

“Traditionally, Japanese would depict a scene emotionally, like ‘The train came out of the long tunnel into the snow country,’ ” Mika Naito, a novelist, said, referring to the famous opening sentence of Yasunari Kawabata’s “Snow Country.”

“In cellphone novels, you don’t need that,” said Ms. Naito, 36, who recently began writing cellphone novels at the urging of her publisher. “If you limit it to a certain place, readers won’t be able to feel a sense of familiarity.”

ImageRin wrote her novel while commuting to her part-time job.Credit...Ko Sasaki for The New York Times