A vertically moving biofilm system (VMBS) was developed to treat wastewater. In this system, the biofilm grows on a biofilm module consisting of plastic media that is vertically and repeatedly moved up into the air and down into the water. The objectives of this study were to investigate the oxygen transfer efficiency and industrial wastewater treatment performance of the VMBS. The oxygen transfer coefficient (K(L)a) depended on the movement frequency (n) of the biofilm module and was proportional to n(1.67). K(L)a values measured were within the range of 0.0001 to 0.0027 s(-1). The VMBS exhibited good carbonaceous removal when treating industrial wastewater produced in a factory manufacturing synthetic fibres. Removal efficiency of filtered chemical oxygen demand (COD) and biological oxygen demand (BOD5) was up to 93.2 and 97.9%, respectively. The volumetric removal rates of filtered COD and BOD5 reached 1320 g COD m(-3) day(-1) and 700 g BOD5 m(-3) day(-1). The areal organic removal rates, based on the surface area of the biofilm substrata, were 16 g BOD5 m(-2) day(-1) and 39 g COD m(-2) day(-1). No clogging occurred during the experiment. The mean areal biofilm mass increased with increasing the mean areal BOD5 removal rate. The new biofilm process has such advantages as high carbonaceous oxidation, energy saving, simple construction and easy operation for industrial wastewater treatment.