Conrad Bain - best known as the kindly adoptive father in the US TV sitcom Diff'rent Strokes - has died at the age of 89.

The veteran stage and screen actor died of natural causes in his hometown of Livermore, California, according to his daughter, Jennifer Bain.

The show that made him famous debuted on NBC in 1978, an era when television comedies tackled relevant social issues. Diff'rent Strokes touched on serious themes but was known better as a family comedy that drew most of its laughs from its standout child actor, Gary Coleman.

Bain played wealthy Manhattan widower Philip Drummond, who promised his dying housekeeper he would raise her sons, played by Coleman and Todd Bridges. Race and class relations became topics on the show as much as the typical trials of growing up.

Coleman, with his sparkling eyes and perfect comic timing, became an immediate star, and Bain, with his long training as a theatre actor, proved an ideal straight man. The series lasted six seasons on NBC and two on ABC.

In the show's heyday, Bain didn't mind being overshadowed by the focus on the show's children. He praised Coleman and Bridges as natural talents without egos. But Diff'rent Strokes is remembered mostly for its child stars' adult troubles.

Coleman, who died in 2010, had financial and legal problems in addition to continuing ill health from the kidney disease that stunted his growth and required transplants. Bridges and Dana Plato, who played Bain's teenage daughter, both had arrest records and drug problems, and Plato died of an overdose in 1999 at the age of 34.

Bain said in interviews later that he struggled to talk about his TV children's troubled lives because of his love for them. After Bridges started to put his drug troubles behind him in the early 1990s, he told Jet magazine that Bain had become like a real father to him.

Bain went directly into Diff'rent Strokes from another comedy, Maude, which aired on CBS from 1972 to 1978.

A native of Lethbridge, Alberta, Canada, Bain arrived in New York in 1948 to study acting after serving in the Canadian army during World War II. He married artist Monica Sloan in 1945. She died in 2009. He is survived by three children: Jennifer, Kent and Mark.