Two l·9 1 Upflow Anaerobic Sludge Blanket (USAB) reactors, operated at 35 °C, were used for the digestion of deproteinated cheese whey substrate. The C/N ratio was 19, and the alkalinity requirements of the two reactors were satisfied by adding Ca(OH)2 and Na2CO3 in two different ratios respectively. Gas and effluent parameters indicated that reactor 1 (receiving a higher level of calcium) was superior and eventually a loading rate of 7·1 kg COD m−3 d−1 could be obtained in this reactor, compared to a loading rate of 5 kg COD m−3 d−1 which was obtained in reactor 2. Pellets with good settling abilities formed in reactor 1, while the sludge in reactor 2 developed a granular slimy appearance and settled poorly. Investigation of the ultrastructure of the sludge particles revealed a difference in the types of bacteria present in the granules.