Studies of the variation in tropical plant species diversity and its relationship with environmental factors are largely based on research in tropical moist/wet forests. Seasonally dry tropical forests (SDTFs) in contrast have been poorly investigated. In this paper we present data from 20 Mexican SDTF sites sampled to describe the magnitude of floristic diversity in these forests and to address the following questions: (i) to what extent is species diversity related to rainfall? (ii) Are there other climatic variables that explain variation in species diversity in SDTFs? (iii) How does species identity vary spatially (species turnover) within the country? We found that species diversity was consistently greater (a ca. twofold difference) than would be expected according to the sites\ precipitation. Rainfall did not significantly explain the variation in species diversity. Likewise the numberof dry and wet months per year was unrelated to species diversity. In contrast a simple measure of potential evapotranspiration (Thornthwaite\s index) significantly explained the variation in species diversity. In addition to the great diversity of species per site (local diversity) species turnover was considerable: of a total of 917 sampled species 72% were present only in a single site and the average similarity (Sorensen\s index) among sites was only 9%. These aspects of floristic diversity and the high deforestation rates of these forests in Mexico indicate that conservation efforts should be directed to tropical forests growing in locations of low and seasonal rainfall.