The invention consists of fermenting selected strains of Blakeslea trispora in conditions such that .beta.-carotene is produced in the form of stabilized preparations with residual contents of other carotenoids (.gamma.-carotene and .beta.-zeacarotene). The fermentation conditions chosen include the programmed addition of oxygen and/or .beta.-ionone during fermentation, and control of the age of the vegetative growth phases of the strains employed. The .beta.-carotene obtained can be used in the pharmaceutical and food sectors.