Vector control agents are now scrambling to control the so-called yellow mosquito's spread.
Tim Phillips of the Fresno Mosquito and Vector Control District says the bugs first were detected in the Central Valley communities of Madera and Clovis in June. They were found this week in the city of Fresno.
The brown mosquitoes with white markings bite during the day, prefer humans and need only a teaspoon of water to lay eggs to reproduce.
So far none of the mosquitoes that have been trapped have carried diseases.
The mosquitoes acquire yellow fever and dengue from infected humans and then transfer them to other people.