The Tehuac & aacute;n-Cuicatl & aacute;n Biosphere Reserve in Mexico, spanning approximately 10,000 km2, is crucial for conserving arid and semi-arid ecosystems, as it hosts unique endemic species and complex ecological interactions. Despite their environmental significance, fungi and myxomycetes in this region have been understudied, particularly those adapted to extreme conditions. These organisms are vital for nutrient cycling, soil stability, and plant health, making them excellent bioindicators for monitoring ecosystem health and detecting environmental changes. However, challenges such as limited historical data, remote fieldwork, and advanced identification techniques complicate their study. Based on a review of mycological literature and various biodiversity databases, the first inventory of fungi and myxomycetes of the Tehuac & aacute;n-Cuicatl & aacute;n Biosphere Reserve (Tehuac & aacute;n Desert) was prepared in this work. This inventory lists 436 taxa of organisms traditionally identified as fungi, belonging to 254 different genera. Of these, 266 taxa belong to 214 genera of fungi sensu stricto, and 170 taxa from 40 genera of myxomycetes. Fungal and myxomycete communities must be documented, and their inherent variability understood through baseline research. Research on fungal adaptation to shifting environments in the Tehuac & aacute;n Valley may reveal resilience mechanisms in desert ecosystems. Fungi and myxomycetes are useful bioindicators for assessing ecosystem health and ecological alterations under global climate stress, due to their rapid environmental response. Understanding these adaptive strategies helps preserve the environment, produce new drugs, and foster agricultural resilience. Polyextremotolerant and extremophilic fungi are studied in the reserve to understand the boundaries of life and survival processes. To use fungi to protect sensitive ecosystems and mitigate climate change in the Tehuac & aacute;n-Cuicatl & aacute;n Biosphere Reserve, interdisciplinary collaboration and innovative methods are needed. The Tehuac & aacute;n Desert can be considered a natural laboratory for studying polyextremotolerant and extremophilic fungi.