Researchers plan to release 30,000 fluorescent mosquitos next month in the second phase of a study of how the insects that spread viral encephalitis have moved from the country to the city.

The potentially deadly disease, which causes an inflammation of the brain and spinal cord, once was thought to be a rural phenomenon but began showing up this decade in populous Southern California. Three blocks of this Los Angeles County suburb have been under study since 1986, when a resident was infected.Urban and suburban outbreaks of viral encephalitis surprised scientists, who want to learn how mosquitoes behave around swimming pools, drainage ditches and landscaping unlike that in rural areas, said Richard P. Meyer of the University of California, Berkeley, School of Public Health.

Previous research has focused on mosquitoes in rural areas, Meyer said.

Researchers will mark 30,000 mosquitos with a fluorescent powder and release them Sept. 7-8 in a Norwalk neighborhood and a park in nearby Downey. They will set about three dozen baited traps around the area, which is next to the San Gabriel River, to try to recapture the mosquitos.

Trapping snared about 5 percent to 10 percent of the more than 30,000 mosquitos released in Rossmoor in neighboring Orange County.

The City Council approved the plan on Aug. 16 despite fears that people could catch the disease. "I think it's wrong that we should be the guinea pig for the universe," said Councilwoman Grace Napolitano.

"The risk of dying is much greater stepping out the front door than from contracting encephalitis from these mosquitoes," said Jack Hazelrigg, of the Southeast Mosquito Abatement District. Mosquitos prefer the flesh of birds and other animals to humans, and the virus hasn't turned up in mosquitos this year.

James P. Webb, an ecologist with the Orange County Vector Control District, said there were no problems or complaints in Rossmoor.