The FBI is investigating the anthrax outbreak
She is believed to be an employee of the same tabloid newspaper publishing company for which two victims worked.
A 63-year-old photo editor died from the disease earlier this month.
The development came as US embassies abroad were told to stock up on the antibiotic Ciprofloxacin as a precaution in case of anthrax attacks.
The condition of the latest victim, a 35-year-old woman, is not known, but she has been hospitalised and is being treated with antibiotics.
She was one of hundreds of people who came forward for testing in Florida after the first anthrax cases emerged.
Americans are still waiting for the results of an investigation into the first two Florida anthrax cases.
The man who died was the first to succumb to anthrax in the US since 1976 - the other victim was found to have spores in his nasal passages.
Computer security experts, for their part, were expected to warn Congress on Wednesday of the possibility of a terrorist attack on systems responsible for communications, power or water.
"To put it simply, we need more people to be doing more creative thinking about computer security. That's what our adversaries are doing," the chairman of the House Science Committee, Sherwood Boehlert, told the Associated Press.
Ongoing investigation
Federal investigators are comparing the strains of anthrax found in Florida against known strains kept at a national repository in Arizona.
Initial tests of the anthrax found in Florida match strains produced for research purposes in the 1950s at a laboratory in the state of Iowa.
Although those strains may have been widely distributed, investigators say that if the match is confirmed, it would rule out the possibility that the men contracted anthrax by natural or accidental causes.
Envelopes containing a white powder have been mailed to about a dozen locations around Florida, but a state health official told The New York Times that they were probably a hoax.
In the wake of US retaliatory strikes against Afghanistan which began on Sunday, government officials have urged police and private security agencies to take extra precautions.
"I encourage all Americans to have a heightened sense of awareness of their surroundings," Attorney General John Ashcroft said.
"I ask them to report suspicious activity to our partners in law enforcement. Every American should be vigilant."
The government officially stands at a Level 3 state of alert, indicating that there is potential for a terrorist attack but no specific and credible threat.
Incidents
A number of incidents since the 11 September attacks have kept Americans on edge.
A subway station near Washington DC was shut down for six hours on Tuesday when a man sprayed a substance into the air and scuffled with police.
Tests showed that the substance was a cleaning agent, but passengers on the train reported feeling ill after the incident.
Local officials say their symptoms could be the result of pepper spray used by officers in subduing Kenneth Ranger, 23, who has been charged with attempted murder of a police officer.
A day earlier, passengers and pilots on an American Airlines flight subdued a passenger who broke into the cockpit during a flight from Los Angeles to Chicago.
The plane, with 162 people aboard, landed safely.
Edward Coburn, 31, has been charged with interfering with a flight crew.
And on Wednesday, a Delta Airlines flight from Atlanta to Los Angeles was diverted after the pilot reported a disturbance in the cabin.