The toxicity of the uranyl ion to the lichen Cladonia rangiferina (L.) Wigg. was shown to be strongly dependent on chemical speciation. Photosynthetic measurements indicated that the anionic complex of oxalate [Formula: see text] was more toxic than the uncomplexed cation [Formula: see text]. No detrimental response could be assigned to the neutral phthalate complex (UO2L). Toxicity was also affected by the physiological condition of the lichen material. Samples exhibiting low photosynthetic levels typical for winter-collected material were damaged to a greater degree. Neither the cationic nor anionic species induced K+ loss from the lichen and the small release induced by the neutral species of the uranyl ion in phthalate buffer reflects a reduction in membrane integrity resulting from Ca2+ depletion. The effects of the uranyl ion and of the buffers used on Ca2+ displacement from lichen samples are discussed. Uranium uptake induced a reduction in total 14C fixation rates, a decrease in the proportion of radioactivity in the ethanol-soluble fraction, and an increase in the ethanol-insoluble fraction. These effects are ascribed to interference with bicarbonate transfer into the algal cell and the blocking of carbohydrate transport from the algal partner.