A female freelance writer who blogged about the pornography industry was threatened with rape. A single mother who blogged about "the daily ins and outs of being a mom" was threatened by a cyber-stalker who claimed that she beat her son and that he had her under surveillance. Kathy Sierra, who won a large following by blogging about designing software that makes people happy, became a target of anonymous online attacks that included photos of her with a noose around her neck and a muzzle over her mouth.

As women gain visibility in the blogosphere, they are targets of sexual harassment and threats. Men are harassed too, and lack of civility is an abiding problem on the Web. But women, who make up about half the online community, are singled out in more starkly sexually threatening terms -- a trend that was first evident in chat rooms in the early 1990s and is now moving to the blogosphere, experts and bloggers said.

A 2006 University of Maryland study on chat rooms found that female participants received 25 times as many sexually explicit and malicious messages as males. A 2005 study by the Pew Internet & American Life Project found that the proportion of Internet users who took part in chats and discussion groups plunged from 28 percent in 2000 to 17 percent in 2005, entirely because of the exodus of women. The study attributed the trend to "sensitivity to worrisome behavior in chat rooms."

Joan Walsh, editor in chief of the online magazine Salon, said that since the letters section of her site was automated a year and a half ago, "it's been hard to ignore that the criticisms of women writers are much more brutal and vicious than those about men."

Arianna Huffington, whose Huffington Post site is among the most prominent of blogs founded by women, said anonymity online has allowed "a lot of those dark prejudices towards women to surface." Her site takes a "zero tolerance" policy toward abusive and excessively foul language, and employs moderators "24/7" to filter the comments, she said.

Sierra, whose recent case has attracted international attention, has suspended blogging. Other women have censored themselves, turned to private forums or closed comments on blogs. Many use gender-neutral pseudonyms. Some just gut it out. But the effect of repeated harassment, bloggers and experts interviewed said, is to make women reluctant to participate online -- undercutting the promise of the Internet as an egalitarian forum.

Robert Scoble, a technology blogger who took a week off in solidarity with Sierra, said women have told him that harassment is a "disincentive" to participate online. That, he said, will affect their job prospects in the male-dominated tech industry. "If women aren't willing to show up for networking events, either offline or online, then they're never going to be included in the industry," he said.

The treatment of women online is not just an equivalent of what happens offline, some women say. The Internet allows the content to be seen immediately, often permanently and far more widely than a remark scribbled on a restroom wall.

"The sad thing is, I've had thousands of messages from women saying, 'You were a role model for me,' " Sierra said in an interview, describing communications she received after suspending her blog. Sierra was the first woman to deliver a keynote speech at a conference on the Linux operating system. Her blog was No. 23 in the Technorati.com Top 100 list of blogs, measured by the number of blogs that linked to her site.

Her Web site, Creating Passionate Users, was about "the most fluffy and nice things," she said. Sierra occasionally got the random "comment troll," she said, but a little over a month ago, the posts became more threatening. Someone typed a comment on her blog about slitting her throat and ejaculating. The noose photo appeared next, on a site that sprang up to harass her. On the site, someone contributed this comment: "the only thing Kathy has to offer me is that noose in her neck size."

On yet another Web site came the muzzle photo, which struck her as if she were being smothered. "I dream of Kathy Sierra," read the caption.

"That's when I got pushed over the edge," she said.