Plant species growing on gypsum substrates, usually found in arid and semi-arid zones, create characteristic landscapes that can harbor highly specialized species, several of which are endemic. In the Cuatro Cie ' negas basin (CCB) gypsum is abundant and creates patches of plant assemblages, but little is known about the floristic dif-ferences between them and the ecological factors driving them. In this study, we describe and compare the floristic composition of twelve fixed plots placed in six different positions on the landscape (five gypsum-rich sites and one with a minimal gypsum concentration, as a control site) within the CCB and the nearby La Vir-gen valley. Species presence was recorded twice a year for two years, the floristic composition of each plot was determined and compared through Beta diversity. A principal component analysis was performed to identify species contribution to the assemblage differentiation and the effect of gypsum and topographic variables were tested. A total of 98 species from 62 genera and 31 families were registered. The results show that gypsum plays an important role in community assembly and differentiation, and within gypsum-rich sites, species assemblages are related to the effect of topographic factors, slope and altitude. However, we suggest that within gypsic soils, more punctual environmental conditions like microclimate, water dynamics, and most likely soil characteristics such as salinity or texture (rockiness), other than gypsum concentration, influence community floristic differentiation.