This invention provides apparatus for the growth of biomass from a supply of suitable nutrient wherein the volume of biomass and the size and shape of the units in which it is present within a vessel (10) in which the reaction takes place can be accurately controlled. Control is achieved by providing a support structure (14) for the biomass comprising one or more movable bodies each of which has an internal reticular structure defining a substantial voidage therein and which essentially fills with the biomass but which does not overfill since excess growth is restricted. Restriction can be achieved by removal of excess biomass or by removal of the bodies at the time that they become filled. The techniques are capable of numerous industrial applications including various biological fermentation processes as well as the purification of sewage and industrial effluents, for example.