The invention provides a process for the production of biomass by culturing a microorganism in an aqueous liquid culture medium circulating in a loop reactor having an effluent gas removal zone where from carbon dioxide-containing effluent gas is removed from the reactor and upstream thereof a degassing zone in which a driving gas is introduced to drive carbon dioxide in the liquid phase into a separable effluent gas phase and having upstream of said degassing zone a nutrient gas introduction zone in which oxygen is introduced into the reactor and mixed with the liquid culture medium therein, characterised in that oxygen introduction in said nutrient gas introduction zone is effected at a plurality of locations along the flow path through said loop reactor at a rate such that the average dissolved oxygen content of said liquid culture medium measured using a polarographic oxygen electrode does not exceed 25 ppm.