UPDATE April 28, 2009: Thanks to Breadteam for pointing out these ants might just as well be native California Harvester Ants (Pogonomyrmex californicus), which have been competing with invasive black Argentine Ants (Linepithema humile).
Red Imported Fire Ants (Solenopsis invicta) are really nasty and you should not attempt to eradicate them yourself. California Dept. of Food & Agriculture has information on their website. Red Imported Fire Ants will aggressively defend their nest, if disturbed, with hundreds of ants swarming out of the colony and stinging the offender repeatedly, injecting venom with each sting.
Considering I inadvertently kicked the nest while walking, the week before I shot these photos, I am inclined to agree with the reader who corrected me that these are more likely to be native California Harvester Ants.
Photo: Diane Edwardson, April 22, 2009. Even after a few extremely dry days, the ants still had a lichen farm growing on the "north slope" of their nest. (Click on photo to enlarge.)
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