So far this year, there have been ten human cases of the virus, from five counties, including Fresno and Merced.
California health officials have not identified the woman, but say she is an 88-year-old woman from Kern County.
Experts are urging everyone to take extra precautions to protect themselves from bites, including using insect repellant when outdoors and getting rid of standing water where mosquitoes breed.
Nationwide, 241 human cases have been reported to the Centers for Disease Control.
