The feasibility of the upflow anaerobic sludge bed process for the treatment of raw potato-maize waste-water (PMW) of chip-processing industry with a high concentration of suspended solids (up to 7 g/l), a high insoluble fr action of COD content (up to 60% of total GOD) and significant quantities of potentially foaming substances, such as proteins and fats, has been demonstrated by operating a 1.8 litre reactor at 35 degrees C. The influent waste strength was 5.3-18.1 g COD/litre, of which the soluble fraction was 3.2-7.4 g COD/litre. The organic loading rate (OLR) achieved in this laboratory-scale reactor was approximately 14 g COD/litre day with treatment efficiencies higher than 75 and 63% on the basis of centrifuged and total COD of the effluent, respectively. Some problems with excessive foaming and sludge flotation, as well as with accumulation of undigested ingredients that occurred at a high OLR (> 10 g COD/litre day) and moderate hydraulic retention times (HRT > 1 day), cart be mostly eliminated by applying shorter HRT (< 1 day, e.g. by recycling some part of effluent) or temporarily decreasing OLR. The preclarification of raw PMW by applying a commercial flocculant led to a better reactor operation at an elevated OLR, though the effluent quality on the basis of centrifuged COD was practically, the same as for the treatment of raw PMW The UASB reactor showed a rapid adaptation to sharp changes in the OLR without significant losses in the treatment efficiency of both wastewaters.