The procedure is applicable to all types of water, except when a large amount of particulate material is liable to be retained by the membrane. The principle covers several steps from selection by applying heat to destroy vegetative bacteria to filtration of the water sample through a membrane filter having a suitable pore size (0,2 µm) to retain the spores in or on it. The filter is placed on a selective culture medium, followed by incubation and counting of black colonies.