The objective of the study was to optimize ergosterol production by Saccharomyces cerevisiae in continuous and fed-batch cultures. Use was made of three optimization criteria: ergosterol yield related to the mass unit of the substrate supplied to the fermenter, and two criteria proposed for the purpose of the study, both incorporating (in addition to ergosterol yield) ergosterol content in yeast cells and proportion of ergosterol in the total D ^5,7-sterols content. In both culture types, specific growth rate was found to affect the content of D ^5,7-sterols in the yeast cells. In continuous culture, three of them (ergosterol, dehydroergosterol, and unidentified sterol) reached their maximum at the dilution rate of 0.74, 0.102 and 0.131 h^-1, respectively. For ergosterol and for the unidentified sterol, these were the dilution rates at which ethanol content in the culture medium began to increase. Dihydroergosterol content was an increasing function of dilution rate. In the fed-batch culture with purely oxidative assimilation of glucose, D ^5,7-sterols content in yeast cells (except that of dehydroergosterol) increased with increasing specific growth rate. Optimization carried out with the three objective functions mentioned above showed that they reached their maxima at essentially the same argument values - both in continuous and fed-batch cultures. This indicates that the ergosterol yield criterion can be substituted for the two, more sophisticated, optimization criteria. The optimum dilution rate for continuous culture was 0.13 h^-1 and the optimum time of fed-batch culture ranged between 6 and 8 hours.