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Ron Palillo -- the actor who played Horshack on "Welcome Back, Kotter" -- died this morning at his home near Palm Beach, FL from an apparent heart attack ... this according to someone close to the actor.

We're told Palillo was found by his partner of many years Joseph Gramm around 4:00 AM. Gramm called an ambulance and Palillo was rushed to the hospital, where he was pronounced dead.

We're told the heart attack was very unexpected. Palillo was 63.

According to one of Palillo's colleagues at G-Star School of the Arts, Palillo had appeared to be in good health ... but was a heavy smoker. We're told he had been suffering from a bad cough and had even scheduled a doctor's appointment for today.

Palillo was known for calling out, "Ooh ooh ooh, Mr. Kotter" as one of the Sweathogs on the show ... which he appeared on with John Travolta from 1975 to 1979.

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After Kotter, Palillo appeared in "Laverne & Shirley" and voiced a character on the Disney cartoon, "Darkwing Duck."