WASHINGTON — The investment industry declared victory Tuesday as the U.S. Supreme Court ruled in a closely watched case that investor advocates had hoped would make it easier to challenge high fees charged by advisers in the $9 trillion mutual fund business.

But industry critics also claimed a win in the case of Jones v. Harris Associates , asserting that the high court decision allows plaintiffs to use the kinds of fee comparisons that could help them prove that advisers’ fees are excessive.