Chromosome numbers of the aggregate species Triglochin maritimum range from the diploid number 2n = 12 to the 24-ploid number 2n = 144. It is shown that the typical Linnaean species from northern Europe is octoploid with 2n = 48 chromosomes, and the same number is typical of the distinct species concinnum from the Pacific Coast of North America. Lower numbers are known from southern Europe, whereas the 20-ploid number has been determined from the Japanese taxon T. asiaticum here elevated to the species level. The segregate from the western plains of North America is shown to be a 16-ploid taxon which has been regarded as a variety of T. maritimum or T. concinnum by various authors; it is here raised to specific rank as T. debile. The eastern North American plant, which may be identical with some southern and western populations not yet studied cytologically, has 2n = 144 chromosomes and is found to be identical with the long ignored species T. elatum. The importance of exact identification and segregation of types differing in chromosome number for all exact geobotanical studies is emphasized.