Using cluster bloc analysis, two separate criteria (the Sprague criterion and the 70 percent criterion) for determining subgroup membership, and two different criteria for stability (100 percent continuity, and agreement a majority of the time) it was discovered that, contrary to the hypothesis, subgroup membership was not stable over time for the natural courts of the Warren Court and the Burger Court through 1978. Small group theory may not be as powerful an explanation of subgroup membership and stability in the Supreme Court as previously believed. (Publisher abstract)