Abstract: There is disclosed a nutrient composition to be added to a hydrophobic medium for culturing microorganisms comprising a micro-emulsion of an aqueous solution of nutrient material in a liquid immiscible with water. As the source of nitrogen, compounds soluble in water are used, such as urea or ammonium nitrate, sulphate or phosphate, and, as the source of phosphorus, compounds soluble in water are used. The medium sould also contain a directly-assimilable source of carbon. Application of this composition for culturing of microorganisms allows operations to be carried out which comprise the degrading of hydrocabons covering an area of water or ground; an application of particular interest for operation is for combatting oil pollution of the sea by biological degradation. As the source of nitrogen, compounds soluble in water are used, such as urea or ammonium nitrate, sulphate or phosphate, and, as the source of phosphorus, compounds soluble in water are used.