In this paper, the authors examine the determinants of import share across several independent regional U.S. markets for portland cement. They show that import penetration is indeed influenced by domestic market structure. They study data from twenty markets covering a six-year period. The result s indicate that import penetration rises with increasing domestic-sel ler concentration. Penetration also rises in periods of strong demand when domestic production approaches capacity constraints. Furthermor e, the authors show that domestic producers face the greatest level o f imports in markets where domestic production is relatively cost ine fficient.