Measuring about one-quarter of an inch, with a black body and white stripes, they're called Aedes aegypti.
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According to a press release from the county, Aedes mosquitos are described as being very aggressive day biters that can transmit diseases such as Zika, Yellow and Dengue fevers.
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Oftentimes found in vegetation and places with standing water, the mosquitos are non-native to California and are usually found in warmer climates.
Contra Costa County officials say now that it's been identified, they'll work to try and prevent the new insect from becoming established locally.
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A job that Tammy Morales and her mother Victoria say they're cautiously hopeful local authorities can do.
They tell ABC7 News that for years, the county has taken multiple steps to help residents fight mosquitoes and other pests.
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"They give you some hanging stuff that you can put around the perimeter. They leave the instructions about standing water, you know, for the larva eggs. But they'll come and treat your yard too. So if you have a big problem with mosquitos, they come out and spray," Morales said.
In the meantime, Contra Costa County is asking residents to be careful and report any day-biting mosquitoes
A task some say, they'll be mindful of in the days and weeks ahead.
"I'm actually going to go buy some off before I get to the house tonight," said Martinez resident, Jason Haggard.