The variability of the levels of atmospheric particulate matter PM10 and its composition is assessed in the rural and urban zones of Mexicali during fall and winter 2008-2009, using a low level volume Minivol sampler, with quartz and Teflon filters. During fall the Mexican norm was exceeded (120 μg/m3 in 24 hr) in only two occasions, while in winter it was exceeded twelve times. The predominant component in fall was geological material with 62.5% for the rural zone and 48.5% for the urban one. In winter the geological material was also predominant with 65% for the rural zone and 54.4% for the urban zone. The aim of this work is the interpretation of the variability of the atmospheric PM10 particulate matter at two selected zones (urban and rural of Mexicali) during fall and winter, with emphasis on the chemical composition. The results suggest that the pollution in the urban and rural zones has common sources during winter whereas in fall the pollution has different sources.