The floras of the tropical vegetation in the Sky Island Sierra la Madera (SMA) near Moctezuma in northeastern Sonora (30°00’N 109°18’W) and the Yécora (YEC) area in the Sierra Madre Occidental (SMO) in eastern Sonora (28°25’N 109°15”W) were compared. The areas are 175 km apart. Tropical vegetation includes foothills thornscrub (FTS) in both areas and tropical deciduous forest (TDF) in the Yécora area. A total of 893 vascular plant taxa are known from these areas with 433 taxa in FTS and 793 in TDF. FTS in SMA and YEC (near Curea) had 220 and 298 taxa, with most of them also in TDF (69.5% and 82.9%). Only 83 taxa in TDF were shared between SMA and YEC (37.7% and 27.9% of the floras). The 49 FTS species in SMA but not YEC were not in TDF either, reflecting biotic influences from the Sonoran Desert (10), southwestern United States (8), Madrean Archipelago (6), and a few from oak woodland and tropical western Mexico. One species (Pseudabutilon thurberi) is endemic to central Sonora and adjacent Arizona. Affinities to the New World tropics are very strong in both areas. The structural dominants that define FTS are widespread, but composition varies greatly locally.