A cyanide-degrading bacteria, Pseudomonas fluorescens NCIB 11764, was used for the treatment of wastewater containing cyanide. After successive subculture for 2 months in cyanide-containing medium, cultures were fully adapted to cyanide and able to grow in a medium containing up to 260 mg l(-1) of cyanide. By fed-batch culture supplying only cyanide, it was confirmed that cyanide was the limiting nutrient as a nitrogen source. Ammonia was found to inhibit cyanide degradation, and the rate of degradation was halved when the ammonia concentration was higher than 1 mM. In a 15-l jar fermenter, 26 mg l(-1) of cyanide was consumed almost completely within 48 h and the final yield of growth on cyanide was calculated as 4.21 mg dry cell wt mg(-1) CN. Cells of P. fluorescens were immobilized on zeolite and applied to a modified activated sludge-type reactor to mimic the actual activated sludge system of a coke-plant wastewater treatment system. In this immobilized system operated batchwise, 26 mg l(-1) of cyanide was degraded completely by 8 g l(-1) of immobilized particles within 10 h. The same reactor system was operated in a continuous mode, and the cyanide concentration of the outlet reached the stationary-state level of less than 0.1 mg l(-1) within 2 days at 0.041 h(-1) of dilution rate and within 3 days at 0.082 h(-1) of dilution rate, respectively.