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The arid and humid zones of Mexico were delimited using the Aridity Index (Al) adopted by the United Nations Environment Programme (Al = Map / Evt). Mean annual precipitation (Map) and evaporation (Ev) data were obtained from the records of 2,165 and 1,750 SMN (Mexico’s weather service) stations, respectively. Potential evapotranspiration (Evt) was calculated using the expression Evt = Ev * 0.8. Both variables (Map and Evt) were interpolated with a resolution of 90 m. Map algebra and Geographic Information System (GIS) spatial techniques were used to obtain the Al, which was classified according to UNEP classes and its population quantified. There were no significant differences between the Evt calculated and the Evt recorded at 30 CLIMWAT-FAO stations (P=0.489). It was found that hyper-arid, arid and semi-arid zones occupy 1,27 and 35% of the country’s total area and are inhabited by 1, 7 and 33% of the population, respectively. The dry-subhumid, moist-subhumid and humid zones occupy 11, 16 and 15% of the country’s landbase and are inhabited by 27, 15 and 16 5 % of Mexico’s inhabitants. This means that 63% of the country has some level of dryness and is inhabited by 41 % of the nation’s population.