The blue fan palm (Erythea armata) oases of the Sierra San Pedro Mártir, Baja California, Mexico, are unique not only because of their occurrence on the Pacific side of the Peninsular Range, but also because they span a steep transition from Mediterranean-type scrubland and piñon woodland to the Vizcaíno and Colorado phases of the Sonoran Desert to the south and east. Here we describe soil-water relations and community composition in 3 grazed Pacific-side palm oases along a 20-km transect from Sonoran Desert at 700 m elevation (El Rincón), to an ecotone of mixed desert scrub, coastal scrub, and mountain chaparral at 900 m (San Miguel), to chaparral and piñon-juniper woodland at 1,200 m (El Represo). Soil water was measured by using a neutron probe during an annual cycle. The palm grove at El Rincón had low soil water in the arroyo and oasis throughout the study period, although water was always present in pools both upstream and downstream. At San Miguel, pools were dry most of the time, but both the arroyo and oasis had moderate soil water, and the arroyo apparently received moisture by subsurface flow from nearby highlands. The seemingly dry oasis at El Represo had moderately high soil water, which was apparently maintained by a local seep. A total of 193 taxa were recorded, but only 30 (15.5%) were common to all 3 sites. In these communities, the Californian component diminished from 40 to 29% across the Californian-Sonoran transition, while Sonoran taxa decreased from 41 to 29% in the opposite direction. Determination of the most important species across this biome boundary revealed a strong Sonoran influence, even in the chaparral-piñon-juniper site. Erythea armata seems to be a species particularly adapted to this arid transitional region.