Air-breathing fish may be unusually resistant to the effects of elevated environmental nitrite ions because they can disassociate oxygen uptake and ion regulation at the branchial surfaces and because the only two species so far investigated (Channa striata and Pangasionodon hypophthalmus), have atypical responses and high tolerance. Here we investigated the effects of nitrite on another air breather, the swamp eel Monopterus albus and report both toxicity and hematological effects including methaemoglobin fraction, hematocrit, hemoglobin concentration, erythrocyte and lymphocyte counts. We found the swamp eel extraordinarilly tolerant with a 96h LC50 of 31.3 mM NO2- in water with 0.3mM Cl-., and no mortality below 25mM. Methaemoglobin rose slowly and reached only reached 35.5% in fish exposed to 25mM. Red blood cell count, hematocrit and hemoglobin concentration were unchanged, even in the highest exposure concentration. Lymphocyte concentrations on the other hand rose significantly during exposure. This study provides further evidence that air-breathers respond differently to nitrite than water breathing fish.