Milfoils (Achillea millefolium agg.) of the Czech Republic were studied in the field as well as in most of the Czech herbaria. Achillea pratensis Saukel et Länger, recently described from Austria, was found to be common in the Czech Republic. Its occurrence was confirmed by chromosome counts (2n = 36) in 74 plants from 26 localities, and more than 240 herbarium specimens of this species were revised. Using canonical discriminant analysis, seasonal variation of A. pratensis was analysed. The first generation (spring) and second generation (summer) shoots proved to be morphologically different (they can be separated mainly by number of nodes, plant height, leaf width, and bract length). Thus, the description of the species provided by Saukel et Länger had to be modified to include spring shoots. Morphological and taxonomic delimitations from other tetraploids (A. collina Rchb.) have not yet been satisfactorily defined. Also, taxonomic relationships to A. tenuis Schur described from Transylvania as well as to A. submillefolium Klokov et Krytzka described recently from the Ukraine remain to be clarified. The phytosociological affinity of A. pratensis in the Czech Republic is documented by 18 relevés. The species is confined mainly to mesophilic meadows of the alliance Arrhenatherion W. Koch 1926, to grasslands such as street and garden lawns which can be classified only to the order Arrhenatheretalia Tüxen 1931, as well as to continental flood-plain meadows of the alliance Cnidion venosi Balátová-Tuláčková 1965. Sympatric occurrence of several Achillea taxa of different ploidy levels was observed at several localities and the presence of natural hybrids (A. millefolium x A. pratensis; 2n = 45) was indicated at two sites by chromosome counts.