Flux can vary along the fiber length in submerged hollow-fiber membranes depending upon the axial gradients of both pressure and foulant layer build-up. However, the measurement of flux is necessarily length-averaged because it is determined by the flow rate exiting the end of the fiber. The length-averaged flux below which no foulant accumulates in a specified filtration time is defined as the critical flux. Critical flux is shown in this work to be a relative rather than absolute value. It depends on the fiber length, observation time, aeration rate and the compressibility of the particles. Fouling will occur in full-scale if the critical flux test is established in tests with fibers that are much shorter than in full-scale and/or with a filtration time that is shorter than in full-scale. (c) 2006 Elsevier B.V. All rights reserved.