It's not clear exactly when the mosquito with West Nile tested positive.
To be clear, a mosquito tested positive, not humans, yet.
But health officials are anticipating a significant year for the virus, especially as temperatures get warmer, and that brings more mosquitos to Los Angeles County.
A 74-year-old Imperial County man died in June after contracting West Nile virus.
He is believed to be the first death caused by the disease in the state this year.
Last year, 11 people died of West Nile in California.