Fix a field k of characteristic zero. If a 1 , . . . , a n (n ≥ 3) are positive integers, the integral domain B a1,...,an = k[X 1 , . . . , X n ]/ X a1 1 + · · · + X an n is called a Pham-Brieskorn ring. It is conjectured that if a i ≥ 2 for all i and a i = 2 for at most one i, then B a1,...,an is rigid. (A ring B is said to be rigid if the only locally nilpotent derivation D : B → B is the zero derivation.) We give partial results towards the conjecture.