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The effects of intense grazing, seasonal drought, and fire on soil microbial diversity (substrate utilization) and activity in a northern Chihuahuan Desert grassland were measured in summer 1997, winter 1998, and spring 1998. Intense livestock grazing was initiated in winter 1995, burning occurred in August 1994, and drought stresses were imposed from October 1994 to June 1997. Microbial diversity was inferred from the cal-bon substrate utilization patterns in both gram (+) and gram (-) Biolog plates. Microbial activity was estimated by the activity of selected enzymes. Neither microbial diversity nor activity was affected by grazing. The interaction of intense grazing and stress sub-treatments only occurred in spring for one set of diversity measurements. The maximum microbial diversity and activity occurred in the winter-drought-stress sub-plots in summer and spring. Burning reduced microbial diversity and most enzyme activities as compared to the control in summer and spring. Microbial diversity was also lower in summer-drought-stress sub-plots than in thr: control in summer and spring. Microbial diversity was highest in summer, intermediate in winter, and lowest in spring. Microbial activity was generally higher in summer and lower in winter. it was concluded that substrate availability was the most important factor affecting the diversity and activity of soil microorganisms within a season. Soil moisture was not the factor causing differences in microbial diversity and activity among the stress treatments, but it was a predictor for some microbial responses under a particular stress.