We compared decomposition of surface and buried, untreated, mixed desert shrub litter to that of insecticide- and fungicide-treated litter. Suppression of fungi reduced decomposition by ~29%; exclusion of microarthropods reduced decomposition by ~53%. Approximately 55% of the organic mass of the untreated litter disappeared during the 6-mo growing season and 23-29% disappeared in the winter months (November through March). There was a consistent pattern of microarthropod colonization of buried litter that was related to the percent organic matter lost. This sequence tydeid mites, trasonemid and pyemotid mites, gamasina and predatory Prostigmata, Collembola and Psocoptera, and oribatids. After 1 yr, large numbers of enchytraeid worms were extracted from buried litter. Decomposition of insecticide-treated litter varied directly with rainfall and soil temperature while abiotic factors accounted for <50% of the variation in decomposition of untreated buried litter. We hypothesize that microarthropods affect litter decomposition in desert ecosystems by inoculating litter with fungal spores, but grazing on fungi, and in a heretofore undescribed mode, by preying on free-living nematodes.