Agaricus bisporus mushrooms were grown in compost amended with either arsenic-contaminated mine waste or an arsenate solution, to a final concentration of approximately 200 microg g(-1). Fungi were cultivated at a small-scale mushroom facility in Vineland (ON), where the controlled environment allowed for a large number of fruiting bodies (mushrooms) to be produced. The total arsenic concentrations as well as speciation were examined for each treatment over several harvests (breaks). Total concentrations were determined by acid digestion and inductively coupled plasma mass spectrometry (ICP-MS) detection and ranged from 2.3 to 16 microg g(-1) dry mass in treatment mushrooms. Arsenic compounds were extracted from mushrooms with methanol/water (1:1 v/v), and separated by high-performance liquid chromatography (HPLC, anion/cation exchange) before detection with ICP-MS. Fruiting bodies from all treatments contained arsenite, dimethylarsinic acid (DMA), and arsenobetaine (AB), and to a lesser extent arsenate and trimethylarsine oxide (TMAO). The ratio of arsenic compounds did not vary greatly over the first three harvests. AB was absent in compost not inoculated with A. bisporus supporting the hypothesis that AB is a product of fungal, not microbial, arsenic metabolism. X-ray absorption spectroscopy results lead us to hypothesize that AB plays a role in nutrient translocation within the fruiting body, as well as maintaining turgor pressure to ensure the mushroom cap remains elevated for maximum spore dispersal.