We assessed the differential response of Azolla microphylla and Azolla filiculoides to fluoride stress by growing them in culture media containing 1 to 50 ppm sodium fluoride (NaF). A. microphylla had a higher total chlorophyll content than A. filiculoides. Both species showed gradual decreases in protein content as the concentration of NaF increased. A. microphylla accumulated less proline than A. filiculoides when more NaF was added to the culture medium. For all concentratons tested, the amount of residual fluoride in the nutrient media that supported A. microphylla was higher than in the nutrient media used for growing A. filiculoides.