For the biological treatment of the wastewater containing highly concentrated thiocyanate, microorganisms for thiocyanate biodegradation were isolated and the biofilm reactor charged with fluidized-carriers of tube chip type was employed. The isolated microorganisms were presumed autotrophs. In a small-scale biofilm reactor for the performance test, the observed maximum degradation rate with 80 vol% of fluidized-carriers was 8.1 kg m(-3) day(-1), which was much higher than those observed in any other reactor systems. The high performance of biofilm reactor was presumed to result from the high concentration of microorganisms attached on fluidized-carriers with high surface area. In a bench scale biofilm reactor for the commercial plant design, the biofilm reactor system showed that thiocyanate of 7000 mg l(-1) was successfully degraded to more than 99.9% of removal efficiency within 36 h of total hydraulic retention time. (c) 2005 Elsevier Ltd. All rights reserved.