1. In the filamentous mold Aspergillus nidulans, net nitrite uptake is inducible by nitrate and nitrite, is probably different from the nitrate uptake system and is partially repressed by ammonium. 2. The concentration dependence of net nitrite uptake by the biA1 facA303 strain of A. nidulans shows a saturation kinetics with an apparent Km value of 0.64 mM. 3. Strains of A. nidulans carrying the nihA1 and nihA1 chlA14 mutations considerably reduced the affinity of the nitrite uptake system for the substrate when nitrite concentrations ranging from 2.5 mM to 10.0 mM were tested (apparent Km values of 5.9 mM and 23.6 mM, respectively). 4. These results suggest that the toxic effect of nitrite on A. nidulans is due to enhanced nitrite uptake by nihA strains when high concentrations are present in the medium.