Utilization of yeast extract and formation of byproduct metabolite were investigated for hyperthermophilic archaeonSulfolobus solfataricus (DSM 1617). In both batch and fed-batch cultivations ofS. solfataricus, maximal cell density, NH4 + ion production and pH change were highly dependent on the ratio of yeast extract to glucose in the medium. Variation of NH4 + ion level was identified as a major cause of pH change during cultivation, and acidification of culture broth was attributed to consumption of NH4 + ions rather than formation of acid byproducts. It was also observed that increase of NH4 + ion concentrations in the medium resulted in greater degree of growth inhibition.