CORPUS CHRISTI, Texas (AP) _ A resurgence of red tide in waters off the Gulf of Mexico is worrying some fishermen, but is not an immediate threat to the public, officials say.

A ban on harvesting shellfish went into effect after the fish-killing microscopic organism was found Thursday in Corpus Christi Bay.

''It was a complete surprise to us,'' said Richard Thompson, director of the health department's division of shellfish sanitation control in Austin. ''We certainly didn't expect it to bloom again this winter.''

An infestation in the fall killed an estimated 22.2 million fish. The toxin given off by red tide is not fatal to humans, although it can irritate the eyes, throat and respiratory system.

The red tide extends across Corpus Christi Bay through Redfish Bay and into the Lydia Ann Channel between Port Aransas to Aransas Bay, department reports indicate.

''If it comes back, man, we're dead over here,'' said Benny Arispe, a 38- year-old fisherman who lives in nearby Fulton with his wife and four children.

Arispe, already suffering financially because of last year's poor season, took his boat out early Friday but was forced to stop dredging when he received a radio message from shore.

Dr. Robert Bernstein, state health commissioner, on Friday ordered the closing of Aransas Bay and adjacent St. Charles Bay to shellfish harvesting effective at 12:01 a.m. Saturday.

Other area bays have not been reopened for shellfish harvesting since the health department closed them in September because of the earlier red tide infestation, Thompson said.

The latest closure coincides with a Texas Parks and Wildlife Department decision to shut down oyster harvesting in all but San Antonio Bay, near the Aransas National Wildlife refuge. The Parks and Wildlife decision, which will be in force from 12:01 a.m. Tuesday until Feb. 20, is intended to protect Gulf waters from overharvesting.

Kirk Wiles of the state Department of Health said the only immediate threat to the public from the resurgent red tide would be from eating oysters, mussels and clams from the affected areas. The red-tide toxins become highly concentrated in these shellfish, he said.

''Fish, shrimp and crabs are perfectly safe,'' Wiles said. ''We're not even talking about the same situation we had this summer.''

Red tide was first detected in the Gulf of Mexico near Galveston in August. Concentrations of flourishing organisms spread steadily south into coastal bays and beyond the Mexican border.

Experts expected the colder weather to kill the organism. But the tide apparently has been able to survive the mild South Texas winter that cooled the bay waters only to 61 degrees.