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Two species of the crustacean amphipod genus Niphargus inhabit the sulfidic groundwaters of the Frasassi caves in central Italy, and both harbor filamentous, sulfide-oxidizing Thiothrix ectosymbionts. As sulfide is toxic to most aerobic organisms, it appeared possible that the ectosymbionts could help their Niphargus hosts with detoxification processes. In this study, mortality due to sulfide was compared between Niphargus individuals with ectosymbionts and individuals whose ectosymbionts had been killed by antibiotic treatment. Both Frasassi- dwelling Niphargus species revealed exceptionally high tolerances to sulfide compared to other amphipod species studied so far. Niphargus individuals without viable ectosymbionts tolerated sulfide levels exceeding those occurring in Frasassi cave waters. Thus, the amphipods may employ Thiothrix-independent mechanisms for sulfide resistance.