Microbiological analysis of 65 samples of molasses from various Czechoslovak sugar works revealed that the presence of thermophilic anaerobic bacteria producing hydrogen sulphide affects the fermentability of molasses and thus their applicability to citric acid production by fermentation. The fermentability of molasses containing bacteria of this group is not increased after pressure sterilization. It was found that hydrogen sulphide inhibits the formation of citric acid. A comparison of the yield of citric acid in substrates sterilized under pressure with substrates partly sterilized by boiling revealed that in most cases sterilization has an unfavourable effect on the production of citric acid.