In this study, the effect of the operator on the clinical performance of amalgam was examined by evaluating the marginal fracture characteristics of four alloys placed by four operators. The results were as follows: --Marginal fracture was influenced by the operator. Therefore, careful attention should be given to the cavity design and the placement technique. --The interaction between operators and alloys was such that, for the worst alloy, there was no difference caused by the operators; for the intermediate alloys, there were large differences caused by the operators; and for the best alloy (a non-gamma 2 system), there was almost no difference caused by the operators. --The difference in marginal fracture characteristics among alloys was greater than the difference attributed to operators. --It appears that all practitioners should use non-gamma 2 amalgams because such amalgams perform better clinically and are less susceptible to variations in operators.