His Little House star Melissa Gilbert tweeted her condolences Tuesday. "This man will be missed," she posted, along with a picture of Bull. "Goodbye Richard, working with you was such a joy but nearly as joyful as being your friend."
Born in Zion, Ill., Bull began his acting career with stage roles at the Goodman Theater in Chicago. He married actress Barbara Collentine in 1948.
In the 1960s, he starred as Doc on ABC's Voyage to the Bottom of the Sea before landing the role of Nels on Little House in the Prairie. Bull played the character from 1974 to 1983.
He is survived by his wife.