A fed-batch operation for high density cultivation of Sulfolobus solfataricus (DSM 1617) in a bench-top fermenter using a feed medium composed of glucose and yeast extract was investigated. The highest maximal cell density obtained in controlled fed-batch cultures was 21.7 g/l. Although higher yeast extract concentrations in the medium favored greater cell biomass yield, cell growth ceased with low cell densities. It was observed that large amounts of inorganic ions, such as sulfate, ammonium, potassium and phosphate ions, were accumulated in the culture broth at higher yeast extract concentrations. This was due to either the addition of the titrant or feeding of yeast extract during cultivation. Fed-batch cultures with additional mineral salts in the feed medium showed much lower cell biomass, indicating that accumulation of inorganic ions has a significant inhibitory effect on the growth of S. solfataricus. Inhibition of cell growth by the presence of mineral ions was further confirmed by the batch culture experiments. Some plausible mechanisms which can account for the growth inhibition at higher mineral ion concentrations have been suggested.