A common type of ultrafiltration (UF) membrane fouling is due to organic gel, encountered in the treatment of various effluents and feed waters to desalination plants; polysaccharides are a major component of such fouling layers. In standard methods, dead-end UF tests under constant pressure are used to determine membrane-fouling propensity, but the results are applied with questionable success to the case of constant flux filtration, favored in practice. Observed differences between fouling resistances from the two modes of UF filtration have not been adequately explained yet. The temporal evolution of specific resistance alpha (considered as the most representative fouling parameter) is investigated herein, aiming to elucidate differences and possible similarities in the different filtration modes. During alginate-solution ultrafiltration, typical gel-type membrane fouling layers are formed with time-varying deposited mass density m (g/cm(2)) depending on permeate flux; thus, the temporal variation of specific resistance alpha, of pressure difference across the fouling layer/cake Delta P-c and of pressure drop per unit depth of cake (the latter proportional to Delta P-c/m) are studied for both constant pressure P and constant flux J operation, within P and J ranges of practical interest. These detailed data suggest that permeation flux is the process variable controlling the development of resistance alpha. Specifically, for constant-flux filtration, the resistance alpha data are well correlated with flux J (as also shown recently), whereas in the constant-pressure mode the commonly observed constant resistance alpha, over a rather broad range of declining flux, is due to the high flux level prevailing in the early stage of filtration. A successful approach is demonstrated to compare and correlate the specific resistance alpha data obtained from the two different filtration modes. The usefulness of the new results in practice is also discussed. (C) 2016 Elsevier B.V. All rights reserved.