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This study was carried out in agricultural soils in La Laguna, Coahuila - Durango, Mexico (annual precipitation 80-250 mm). The structure of soil fungal genera of three field soil was studied. In soil of Tierra Blanca, an orchard pecan of 51 years, there were isolated 23 genera of soil fungi, and only 12 genera of other two field soils (El Chupon, a pecan orchard of 14 years, and San Jorge, culturing with alfalfa). For each genus of soil fungi there were calculated the Importance Value Index (I V I). The genus Fusarium showed the greater I V I of the three field soils (71-98). The diversity (Shannon’s index) of genera of fungi was different from each soil according to confidence intervals (95%), with values of 1.89, 1.72 y 1.19 for Tierra Blanca, El Chupon and San Jorge, respectively. Additional ecological indexes were calculated, as Simpson, maxim richness (H’ max), regularity (F). The values of Index Simpson and F were similar between Tierra Blanca and San Jorge, but only H’ max was similar between El Chupon and San Jorge. The similarity index of Shoresen was equal for Tierra Blanca vs. El Chupon, or Tierra Blanca vs. San Jorge (51.4), whereas El Chupon vs. San Jorge was 58.3. The index B for the genera of fungi in Tierra Blanca vs. El Chupon or Tierra Blanca vs. San Jorge was 0.48, but San Jorge vs. El Chupon was 0.83. The genera of soil fungi found in La Laguna are 67-75% similar to soil fungi isolated from desert, such as the Israel desert, but in soils of La Laguna the structure of genera fungal is different; here Fusarium is the most abundant, and appear new genera such as Trichoderma. The changes of soil mycobiota could be the result of the effect of agricultural activity during the last 50 years.