Florence Latimer Mars, who defied the society into which she was born to write a searing book about the effects of the 1964 killings of the civil rights workers Andrew Goodman, Michael Schwerner and James Earl Chaney on her hometown, Philadelphia, Miss., died on Sunday at her home there. She was 83.

The author of "Witness in Philadelphia," published in 1977 by the Louisiana State University Press, she repeatedly spoke out against the Ku Klux Klan and other forces oppressing the black population of east central Mississippi. A fourth-generation resident of the area and a member of its landed gentry, she was also a significant source of information for the F.B.I. agents investigating the killings, and she testified before a federal grand jury.

Miss Mars paid dearly for her efforts. The Klan organized a boycott against the stockyard where she sold cattle, forcing it to close, and she was compelled to resign from posts at the First United Methodist Church.