The genus Parietaria L. in the Netherlands, 1 & 2. A study of herbarium specimens and of living plants of both species of Parietaria occurring in the Netherlands permits the following conclusions: 1. Both species can not possibly be conspecific on account of the constant differences in the morphology of the inflorescence, in the chromosome number, and in their periodicity, ecology and distributional areas. 2. The binomial Parietaria judaica L. 1756 has priority over the name P. diffusa proposed by Mertens & Koch in 1823, and the illegitimate name P. ramiflora Moench 1794. 3. The two species can be distinguished as follows: P. officinalis L., Sp. Pl., 1753, p. 1052. Syn.: P. erecta Mert. & Koch 1823; P. officinalis L. var. erecta (Mert. & Koch) Weddell 1857; P. officinalis L. subsp. erecta (Mert. & Koch) Béguinot 1908. Stem usually erect, branched or unbranched, (20—) 50—70 (—160) cm tall, 3—6 mm in diam., as a rule scantily pubescent, green or faintly erubescent, hollow; in the autumn with red hibernation buds. Leaves comparatively large, (5—) 8—12 (—20) cm long, ovate-oblong, often long-acuminate; the blades from about 3 to about 5 times as long as the petiole. Inflorescences almost invariably multiflorous, forming dense clusters of flowers. Bracts of the flowers of the cincinnate portions of the cymes free at the base. Perigone after anthesis not or hardly accrescent, 2 mm long, closed around the 1,5 mm long nutlet. Haploid chromosome number: n = 7. P. judaica L., Fl. Palaest., 1756, p. 466. Syn.: P. ramiflora Moench 1794, nom. illeg.; P. diffusa Mert. & Koch 1823; P. officinalis L. var. diffusa (Mert. & Koch) Weddell 1857: P. officinalis L. var. ramiflora (Moench) Aschers. & Graebn. 1911; P. officinalis sensu Jarmolenko 1941, non L. Stem usually ascending or prostrate, frequently ramified, (3—) 20—40 (—110) cm long, 2—3 mm in diam., usually densely pubescent, darkred, pithy; not forming hibernation buds (in springtime new shoots are produced from the rhizome). Leaves comparatively small, (1—) 4 (—6) cm long, ovate, often shortly acuminate; the blades from about 2 to about 3 times as long as the petiole. Inflorescences usually pauciflorous, forming small lax clusters. Bracts of the flowers of the cincinnate portions of the cymes connate at the base. Perigone manifestly accrescent after anthesis, 3 mm long, slit open; nutlet 1 mm long. Haploid chromosome number; n = 13. 4. It seems highly probable that a tetraploid form of P. officinalis or a closely allied taxon must have been one of the parent forms when the taxon P. judaica originated. 5. P. judaica exhibits a marked preference to walls as a habitat, whereas P. officinalis prefers growing under shrubs or in other sheltered (and often ruderal) sites. 6. Of the two, only P. judaica is a species of mediterranean-atlantic distribution; the principal area of distribution of P. officinalis lies presumably in the Danube countries of central Europe and in the Balkans. 7. P. judaica may be considered to be truly native in this country in the so-called Fluviatile District, but P. officinalis is probably an escape from cultivation.