A continuous-flow fluidized-bed bioreactor system containing porous carrier particles was operated for simultaneous removal of carbonaceous and nitrogenous substances from wastewater. The bioreactor system consisted of two aerobic reactors in series with recycle-flow. Organic oxidation, ammonification of urea and nitrification took place in both reactors, while denitrification concurrent with those reactions occurred in the reactor where the artificial sewage and recycled fluid were fed. In the steady state, measurement of microbial activities in the reactors confirmed the presence of the respective bacteria participating in the above reactions, These multiple reactions proceeded simultaneously in the bioreactor system were analyzed by a set of simple reaction kinetics, which interpreted the experimental results well.