Giardia lamblia is an amitochondrial protozoan susceptible to oxygen, but the molecular basis for it remains unclear. A Giardia NAD(P)H:menadione oxidoreductase (DT-diaphorase) is known to catalyse a single electron transfer reaction with quinones as the likely two-electron acceptor when oxygen is absent. Here we overexpressed this enzyme in Giardia trophozoites and observed a significantly enhanced susceptibility of the cells towards oxygen. A knock-down of this enzyme resulted, however, in more oxygen-tolerant Giardia cells growing equally well under anaerobic and aerobic conditions. The function of DT-diaphorase could be thus a major, if not the only, cause for the oxygen susceptibility of Giardia. Overexpressed DT-diaphorase is accompanied by increased intracellular hydrogen peroxide. An overexpression of Fe-superoxide dismutase in Giardia led also to a similarly heightened sensitivity to oxygen. Thus, generation of H2O2 from superoxide anion likely produced from DT-diaphorase catalysed reaction using oxygen as electron acceptor may constitute the molecular basis for Giardia susceptibility to oxygen. A functional homologue of DT-diaphorase in Giardia, NADH oxidase, uses oxygen as the preferred electron acceptor and reduces it to water. Overexpression of this enzyme in Giardia resulted in significantly enhanced growth under aerobic conditions. Giardia NADH oxidase could be thus an instrumental enzyme for the organism to adapt to and to tolerate an aerobic living environment.