Anaerobic biological removal of chlorate was studied on a laboratory and pilot plant scale. Continuous laboratory tests in an anaerobic fixed-film process showed that chlorate can be removed completely from kraft bleach effluent at such a short hydraulic retention time as 0.6 h. The efficiency of biological chlorate removal was confirmed under practical conditions in a 20 m3 pilot plant, operating at a Swedish craft mill. Four chlorate reducing bacterial strains were isolated and characterized. All four isolates were gram-negative, catalase- and oxidase-positive, motile rods. None of the four isolates could ferment glucose, while they could all grow aerobically and with nitrate as electron acceptor.