Abstract: A fermentation apparatus for use with liquid substrates comprises a housing having arranged therein a shaft of an agitation means provided with stirrers. Arranged in the interior of the housing are two groups of elements intended for the cultivation of layers of microorganisms thereon, the elements acting to expand the useful interior area of the apparatus, the two groups being adapted to form a gap therebetween. At least one group of the elements is adapted to be moved relative to another group to thereby control the thickness of the layer of the microorganisms being cultivated in said gap during the movement.