Anthropogenically caused physical and chemical habitat degradation, including water pollution, have caused dramatic declines in freshwater mollusk populations. Sodium dodecyl sulfate (SDS), a surfactant with no USEPA Water Quality Criteria (WQC), is commonly used in industrial applications, household cleaners, personal hygiene products, and herbicides. In aquatic habitats, previous SDS studies have associated deformities and death to mollusks found in these systems. The objective of this study was to determine EC₅₀ values for two freshwater juvenile unionids (Villosa nebulosa and Hamiota perovalis) and two freshwater caenogastropods (Leptoxis ampla and Somatogyrus sp.) endemic to the Mobile River Basin, USA, to SDS. Using the Trimmed Spearman-Karber method, EC₅₀ values were calculated. Results found that EC₅₀ values were: V nebulosa = 14,469 µg/L (95% CI: 13,436 – 15,581 µg/L), H. perovalis = 6,102 µg/L (95% CI: 4,727 – 7,876 µg/L), Somatogyrus sp. = 1,986 µg/L (95% CI: 1,453 – 2,715 µg/L), and L. ampla = 26 µg/L (95% CI: 6 – 112 µg/L). Freshwater gastropods were more sensitive to SDS than freshwater unionids. Leptoxis ampla was the most sensitive species tested and had such a low EC₅₀ value that more protective regional criteria may be required. Therefore, future research should include additional testing on mollusk species, particularly regionally isolated species that may display increased sensitivity.