The acidophilic red alga, Cyanidium caldarium Geitler, could use nitrite as a nitrogen source for growth, although this compound was very toxic in acidic media. Growth could be sustained when nitrite was added continuously at a rate lower than the maximum rate of nitrite assimilation for the culture. Nitrite assimilation was derepressed in cells growing on nitrate or nitrite, under nitrogen limitation and by nitrogen starvation. Ammonium-grown cells showed a limited capacity for nitrite reduction in the light, even if nitrite reductase was not detectable in cell extracts. In acidic media nitrite was taken up mainly through HNO, influx. Nitrite assimilation, in viva, was restricted by a saturable step under conditions in which nitrite uptake was not limiting. The pH of the cell suspension affected nitrite assimilation by changing the K½ over the pH range tested but not the Vmax. The K½ was inversely proportional to the concentration of H+ in the medium. The apparent Km value for nitrite of nitrite reductase, in vitro, and its expected apparent Km value, in vivo, estimated from the K½ of nitrite assimilation, suggested that the activity of this enzyme may not be the limiting step of nitrite assimilation.