In lethal and sublethal ammonia toxicity tests, we examined differences in tolerance of three species of freshwater amphipods, one native and two invasive in Ireland. The native Gammarus duebeni celticus was slightly less tolerant to ammonia than the invasive G. pulex (96h LC₅₀ = 1.155 and 1.544 mg l⁻¹, respectively), while another invader, Crangonyx pseudograeilis, had the lowest tolerance (LC₅₀ = 0.36 mg l⁻¹). Parasitism of G. pulex by the acanthocephalan Echinorhynchus truttae greatly reduced the tolerance of the invader to ammonia (LC₅₀ = 0.381 mg l⁻¹). Further, precopula pair disruption tests indicated that G. d. celticus was more sensitive to ammonia than G. pulex at sublethal levels. We discuss these results in the context of the ecological replacements of native by invader amphipods.