FORT COLLINS, Colo. -- Locked in a visitation dispute with his ex-girlfriend over their young daughter, J.P. Weichel wanted to vent, court records show.

Mr. Weichel, 40, allegedly posted comments about the woman on the Craigslist "Rants and Raves" forum, accusing her of child abuse and welfare fraud and making crude comments about her sex life.

The woman said the postings were defamatory. But unlike the vast majority of libel cases, which are tried in civil court, local authorities have taken the unusual step of charging Mr. Weichel with the crime of defaming his ex-girlfriend online.

Colorado is one of a dwindling number of states with a criminal law against libel. The statute dates to the 19th century and is rarely used. But Larimer County District Attorney Larry Abrahamson said Colorado's statute applied precisely to what Mr. Weichel did. "This is what the legislature of the state of Colorado has determined is criminal," he said. "We're obligated to enforce the laws in the state of Colorado."

Mr. Weichel could not be reached for comment. His attorney, Michael Liggett, has a policy of not speaking to the media, said an assistant at his law office.

Several other lawyers said the case should be handled in civil court. Bringing the government into the dispute infringes on free speech, they said. "Being a jerk isn't necessarily grounds for felony prosecution," said Mark Silverstein, legal director of the American Civil Liberties Union of Colorado.

Georgia State University communications professor Gregory Lisby has tracked criminal libel prosecutions. He said the states that retain such laws -- there are 17, according to free-speech groups -- simply have failed to update legislation that stems from 15th-century English common law. Criminal libel prosecutions are "a sledgehammer when a scalpel would do the same trick," he said.

Mr. Lisby's research has shown criminal libel prosecutions dropping, but the Internet could reverse that, he said. People don't realize that scathing postings or e-mails can make them liable for defamation charges. "More and more people view the online world as a free-rant place," he said. "They think it's par for the course, but they're setting themselves up for lawsuits" or prosecution.

In civil libel cases, truth is the best defense, and the dead cannot be libeled. But Colorado's criminal statute holds that it is illegal to "blacken the memory of one who is dead." Truth is not a defense in those cases, nor in ones that "expose the natural defects of one who is alive."

The state Supreme Court upheld the law in 1991, but warned that its provisions would not apply to constitutionally protected political speech. In 2006, the ACLU successfully blocked Greeley, Colo, police from using the statute to pursue a blogger who posted material critical of a University of Northern Colorado professor. The ACLU argued that prosecution would violate the First Amendment.

According to police and court records, Mr. Weichel's postings were purely personal. His ex-girlfriend last December contacted police in Loveland, Colo., about anonymous postings that referred to her sometimes by her nickname and other times by her full name.

According to a police report, the postings alleged that she abused her child and concealed it from social workers, that she committed welfare fraud and that she worked for a "crooked" Fort Collins lawyer whom she sexually serviced.

The postings were laced with crude references to her body. The woman told police that people who knew her read them and tried to defend her in online comments.

Police traced the postings to a computer to which Mr. Weichel had access. In August, Loveland police questioned him at his workplace about the postings. "Weichel stated he was 'just venting,' " according to an affidavit for an arrest warrant filed in court in Fort Collins.

Mr. Abrahamson's office filed two criminal libel charges against Mr. Weichel on Oct. 21. The state's libel law carries a maximum sentence of 18 months in jail.