This paper presents the results of a study on sulfate resistance of plain and fly ash roller compacted concretes (RCCs). A total of 12 plain, 24 cement-replaced, and 12 fine aggregate-substituted fly ash concretes were used in this investigation. Laboratory-made RCC specimens were prepared at their optimum moisture content and were fabricated in accordance with ASTM C 1170, Procedure A. The test samples were initially moist-cured for 28 days after casting, prior to immersion in a 5% sodium sulfate solution. Length change, mass loss, and compressive strength were monitored for a period of 180 days to evaluate the performance of specimens exposed to very severe sulfate attack. The influence of mixture variables (cement, coarse aggregate, and fly ash contents) on bulk characteristics and sulfate resistance were evaluated. The study shows that good sulfate-resistant RCCs can be attained with the use of Type V portland cement with or without low-calcium fly ash. The resistance to sulfate attack improves with increases in cement or coarse aggregate content, as concrete becomes more dense and impermeable. Length change of RCC samples increases with increasing immersion age and stabilizes within 3-4 months after the initial contact. No mass of concrete residues is found for any specimens tested in this study. However, after 6 months of immersion in a sodium sulfate solution, RCC mixtures with cement content of 12% or less (by mass of total dry solids) experienced slight reduction in strength. A 20-40% replacement of cement by low-calcium fly ash increases the sulfate resistance of RCC samples (excluding mixtures made with 9% cementitious binder and 20% fly ash), whereas 10% replacement has a contrary effect. Mixtures with 10-20% fine aggregate-replaced Class F fly ash exhibit lower sulfate expansion and higher compressive strength than those of plain and cement-substituted fly ash RCCs.