A study was undertaken to determine the influence of nitrogen application on the production of basidiocarps and diversity of the Agaricales s.l. in a Nothofagus obliqua forest of Chile. Six plots were established in the studied area: four plots were fertilized with 10 g m^-2 of NH₄NO₃, and the others were used as controls (unfertilized). There basidiocarps were collected and counted; also microclimatic and soil chemical properties were measured during one year from March 2002. The highest number of saprotrophic, ectomycorrhizal and parasitic species of Agaricales s.l. was found. However, the number of taxa and basidiocarps depended on the concentration of NO₃. In the control plots (3.400 mg kg^–1 of NO₃), 12 ectomycorrhizal species (with an average of 49 basidiocarps), 28 saprotrophic species (average of 146 basidiocarps) and one parasitic species (average of 7 basidiocarps) were found. Fertilized plots (9.350 mg kg^–1 of NO₃) had three ectomycorrhizal species (average of five basidiocarps) and 18 saprotrophic species (average of 96 basidiocarps). Cortinarius vaginatus was the only ectomycorrhizal species found in fertilized plots. Five species previously unknown to the N. obliqua forests of Chile were found: Cortinarius pseudoclaricolor, C. vaginatus, Inocybe geophyllomorpha, Mycena austroavenacea and M. sanguinolenta. Nitrogen fertilization treatment decreased diversity and density of Agaricales s.l. in N. obliqua forest.