Utilization of yeast extract and formation of byproduct metabolite were investigated for hyperthermophilic archaeon Sulfolobus solfataricus (DSH 1617). In both batch and fed-batch cultivations of S. solfataricus, maximal cell density, {{{{ { NH}`_{4 } ^{ +} }}} ion production and pH change were highly dependent on the ratio of yeast extract to glucose in the medium. Variation of {{{{ { NH}`_{4 } ^{ +} }}}} ion level was identified as a major cause of pH change during cultivation, and acidification of culture broth was attributed to consumption of {{{{ { NH}`_{4 } ^{ +} }}}} ions rather than formation of acid byproducts. It was also observed that increase of {{{{ { NH}`_{4 } ^{ +} }}}} ion concentrations in the medium resulted in greater degree of growth inhibition.