Maybe there was something amiss in the hiss.

Maybe it was viperous vibes.Whatever the serpentine signal he received, Tom Robertson Jr. deemed things were not as they sssssseemed.

Robertson, 36, a self-taught snake expert and former zoo employee, walked into a pet store on New York City's Upper West Side about a week ago to buy some fish food.

When he glanced at a tank that supposedly held harmless garter snakes, he recognized similar-looking Asian water snakes known as Redneck Keelbacks. The snake's bite, in rare instances, can be fatal.

"I told a guy in the store," said Robertson. "He said he knew they weren't poisonous. I told him I could bring them some proof; he got real arrogant. I brought back a book, but they still didn't believe me. . . . I called the police, the ASPCA, Fish and Wildlife - I called a lot of places, trying to find out what to do."

Finally, someone at the city Health Department took him seriously.

The department's inspectors called a New York University biologist who confirmed Robertson's suspicions.

The inspectors learned that a wholesaler had sold Petland Discount stores about 50 supposedly harmless snakes - and that some had been distributed to the chain's 50 stores in the metropolitan area.

The Health Department issued a public warning about the snakes Friday and is investigating the matter.

"They believe me now," said Robertson, who received congratulations Friday from neighbors and relatives.