The research material consisted of 802 relevés recorded during 2002–2004 using the Braun-Blanquet method in plant communities of meadows and pastures and in the neighboring marshes, xerothermal swards, as well as water and synanthropic plant communities. The floristic diversity was assessed at the species and plant-community levels. Attention was paid to the share of species of grasses, papilionaceous and honey plants, medicinal herbs and protected and rare plants, as well as those threatened by extinction, in the flora of the studied area. In plant communities, in which minimum 5 relevés were recorded, each time on/from the area of 100 m2, the total number of species was counted, the average number in the relevé and the Shannon-Wiener’s diversity index were calculated. A general, natural-scientific valorization of the San river valley was carried out on the basis of floral species composition and numerical data of analyzed plant communities on sample areas. Additionally, attention was paid to differences in bio-diversity of syntaxa identified within the Arrhenatheretum elatioris and Lolio-Cynosuretum associations. These associations occupied the largest area among the meadows. Phytosociological studies led to identifying 80 phytocoenoses at the association and community ranks, which were classified to 11 classes. Phytocoenoses of the Molinio-Arrhenatheretea and Phragmitetea classes dominated. The flora of the described non-forest communities consisted of 591 species of vascular plants, including over 30% of rare taxa, protected and/or threatened by extinction on the regional and national scale. Particularly valuable are the plant communities in marshes and xerothermal swards, which are threatened by extinction because of changes in habitat conditions and changes in the manner and intensity of utilization, among others. Only 35% of the identified phytocoenoses are not under threat. Species particularly threatened in that area comprise: Trapetum natantis, Caricetum paniculatae, plant community with Fritillaria meleagris, among others, which occurred on single stands only. Floral species composition and the richness of plant communities on meadows, rushes and others of nonforest areas have confirmed clearly that the San river valley is of high value for natural science. The value of the Shannon-Wiener index was diversified (H = 1.29-3.54) and usually lower for plant communities of Phragmitetea class in water and rushes, and higher for phytocoenoses of the Molinio-Arrhenatheretea class.