Studies were made of the aquatic liverwort Scapania undulata (L.) Dum from streams in the Polish Sudeten Mountains. It was found that this species growing in water with mercury concentrations below the detection limit (10(-3) micrograms/liter) contains from 0.025 to 4.7 mg Hg/kg dry wt. According to the bioassay the lethal concentration of Hg2+ (24-hr LC100) for S. undulata appeared to be 500 or 1000 micrograms/liter, depending on the population the liverworts were chosen from. Exposure to 16 micrograms Hg2+/liter for 14 days may be recognized as harmless for S. undulata. After completion of the experiment S. undulata, treated with 64-512 micrograms Hg2+/liter, accumulated respectively 46-2800 times more mercury than the blank.