In a previous paper (VAN DER MAAREL, 6) the results of an investigation of herbarium specimens of Cochlearia officinalis and C. anglica as to their distinguishing characters have been published. Now similar measurements of populations of these two species are described of which the results are shown in fig. 1, a-d and table I. It appears that the so obtained averages are very near those found in measurements of herbarium specimens. Table II gives the 1% confidential limits of those averages. Characters a, d and f seem to be the most useful ones. The occurrence of intermediate populations is considered by the author as a result of introgressive hybridization. The importance of the phenomenon for the study of some aspects of plant taxonomy, ecology and sociology (especially study of the alliance Agropyro-Rumicion) is emphasized. Some suggestions for modern interpretation are given. C. officinalis and C. anglica should be termed ecospecies sensu Turesson, forming a commiscuum sensu Danser, which may be regarded as a dysploidion sensu Camp & Gilly. Intermediate forms/ populations could be termed C. cline officinalis-anglica (after Huxley); determination of hybrid index (Anderson) should be carried out. Information is required about the occurrence of hybrid swarms not only from the Netherlands, but also from abroad.