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.