Domoic acid has been detected in shellfish in Monterey and Santa Cruz counties, according to the California Department of Public Health.
The acid leads to an illness known as amnesic shellfish poisoning.
There are no known cases of deaths due to poisoning from domoic acid in California, according to state public health officials.
The warning applies to certain types of seafood such as mussels, clams, oysters or whole scallops, which fall under the category of bivalve shellfish.
Commercial bivalve shellfish are not under the warning since they are sold by approved sources subject to frequent mandatory testing for toxins, state public health officials said.
Symptoms of domoic acid poisoning include vomiting, diarrhea, abdominal cramps, headache and dizziness. Severe symptoms include trouble breathing, confusion, disorientation, cardiovascular instability, seizures, excessive bronchial secretions, permanent loss of short-term memory, coma or death.
Symptoms can occur within 30 minutes to 24 hours of consuming toxic seafood.
More information on shellfish poisoning and quarantines can be obtained by calling the CDPH's toll-free "Shellfish Information Line" at (800) 553-4133.