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Wadi Helo (WH), in the United Arab Emirates, is a famous archeological site and protected area in the Hajar Mountains in the Arabian Peninsula. We investigated effects of topography, aspect, and altitude on floral diversity and species composition in this heterogeneous, mountainous, and highly arid environment. Using twenty transects distributed from the mountains top to the wadi bed; eighty plots, 100 m(2) each, were selected for vegetation analysis. Classification of vegetation using TWINSPAN showed three groups in the wadi bed and five in the mountains, each distinguished by different dominant species. Forsskaolaea tenacissima, Rhazya stricta, Tephrosia apolina, Notoceras bicorne, Acacia tortilis, and Fagonia indica were dominants of the microhabitats of the wadi bed. Gorges were dominated with Dodonaea viscosa and Lavandula subnoda. The other four groups were found on the mountain slopes and were associated with particular aspects (being north-, south-, west-, or east-facing). The dominant species for these different aspects were, respectively: Boerhavia elegans; Ochradenus aucheri and Helianthemum lippii; Moringa peregrina; and Euphorbia larica and Cenchrus ciliaris. Altitude showed highly significant positive relationship with species frequency (P < 0.001). South-facing slopes had the highest species richness and concentration of dominance. We concluded that the distribution of dominant species was a reflection of their ecological requirements and adaptations. Seven plant species were recorded as new to the flora of the UAE. These, plus ten additional species that had been reported as rare in only one of the two published floras of the UAE were highlighted as meriting greater priority in conservation.