Longlasting investigations have been performed in a swimming pool, which has been the source of a pathogenic agent responsible for 16 fatal cases of primary amoebic meningoencephalitis. A description is given of the methods employed for confirming, by cultivation and microscopy, the presence of potential pathogenic strains of amoebae of the limax type in the water and in all areas of the swimming pool moistened by the water from the pool and also of methods employed for testing the pathogenicity of the isolated strains. Quantitative cultivation examination revealed that the concentration of amoebae capable of growth at 37°C in water treated with the prescribed amount of free chlorine and answering to all bacteriological and chemical norms, ranged from 102–103 specimens in one liter of water. The principle site of amoebic reproduction are the walls and the floor of the pool mainly in its deeper parts, where the density of protozoan populations may reach several thousands per 1 cm2. Also the large inner surface of the sand filter offers most favourable conditions for amoebic reproduction. No direct relationship has been observed between the occurrence of amoebae and the bacteriological and chemical state of the water. The presence of 11 species of amoebae of the limax group has been identified in the pool including several strains of the species Naegleria gruberi. The moderate pathogenic capability of only 5 strains of the species Acanthamoeba castellawi and A. polyphaga has been confirmed in experimental infection of mice. Pathogenic strains of the species Naegleria gruberi, the etiologic agent of primary amoebic meningoencephalitis of man, has not been found either in the pool itself or in the adjoining area. Various control measures have been suggested which may reduce the density of amoebae of the limax type in the pool and, hence, reduce the danger of infection with primary amoebic meningoencephalitis acquired through bathing.