A strain of Bacillus firmus (designated strain KC) isolated from a boron (B) mine in California exhibited extreme tolerance to B, provided it was first acclimated at intermediate B supply concentrations. Strain KC tolerated up to 1000 mmol/L B (boric acid-B) and 1800 mmol/L B (sodium tetraborate-B), and attained the greatest growth (as measured by absorbance) at 300 mmol/L B. Despite its extreme tolerance to high B, there was no evidence that it was able to remove significant quantities of B from the growth media, suggesting that strain KC is not likely to be useful for the removal of B from wastewaters in an engineered bioreactor.