This International Standard describes a monitoring method for the isolation and enumeration of Legionella organisms in water intended for human use (e.g. not and cold water, water used for washing), for human consumption and for treated bathing waters (e.g. swimming pools). It is escpecially suitable for waters expected to contain low number of Legionella. As the growth of Legionella may be inhibited by overgrowth of other bacterial colonies on the membrane, the method is only suitable for waters containing low bacterial counts.