A study was made of the chromosome number of a population of Cochtearia pyrenaica DC. occurring near Moresnet in NE. Belgium. The diploid number appears to be 12; no accessory chromosomes were observed. The plants studied are characterized by very large heterochromatic regions which are responsible for the appearance of a conspicuous zonation-pattern in the chromosomes during late anaphase. The possible significance of this phenomenon in the taxonomy of the Cochtearia is discussed.