The floristic composition and species diversity of Brassica nigra communities were investigated in Beni Suef Governorate, Egypt. In 46 stands, a total of 49 species belonging to 42 genera and 18 families were recorded. Vegetation classification and ordination distinguished seven groups: two in reclaimed land only, three groups in old cultivated land only, and two that occurred in both types of land. The species that dominated these groups were Brassica nigra, Sonchus oleraceus, Beta vulgaris, Cichorium endivia, Euphorbia helioscopia and Anagallis arvensis. The highest species diversity was mainly in groups from reclaimed land, and in wheat compared to other crops. Edaphic factors, especially soil texture, CaCO 3 and organic carbon, contributed significantly to explaining the distribution of some weed species, but not with that of B. nigra. The allelopathic potential of Brassica nigra reported in previous studies did not seem to play a role in community composition.