Arsenate was accumulated by discs of the lichen Umbilicaria muhlenbergii (Ach.) Tuck, to levels of 16/wnol As per gram oven-dry weight during 1 h incubation in solutions containing 40μmol of arsenate at pH 4–6. Studies at lower external concentrations and at different pH values revealed that the more protonated species of arsenate were absorbed most readily. Light enhanced arsenate uptake only marginally, which supports the interpretation that the bulk of uptake was by the non-photosynthetic fungal symbiont. Dinitrophenol caused a 30% reduction in uptake, while heat-killing lichen samples resulted in negligible absorption, even from solutions of high arsenate concentration. The kinetics of arsenate uptake were studied using the Michaelis-Menten treatment, and yielded an apparent value for Km of 360 ± 60 μ and a Vmax of 0.37 ± 0.03 μmol g−1 min−1 for H2AsO−4. Fürther analysis by means of the Hofstee and Lineweaver-Burk plots suggested that two uptake systems may exist. The above results are discussed in relation to published data on free-living fungi and algae and it is concluded that arsenate uptake provides a convenient means for investigating anion accumulation in lichens.