The myrmecofauna of Bulgaria is diverse, including Central and Northern European species as well as thermophilic and xerophilic Mediterranean species. Potential corridors for thermophilic fauna penetration include the valleys of the Struma, Mesta, and Maritsa rivers. Here, we consider the species diversity and the availability of thermophilic species in the Mesta river valley. Three sampling plots were situated in the Mesta river biocorridor, from the southernmost point in the Bulgarian territory up north to where Mediterranean influence is still significant. A total of 30 pitfall traps and 2 mesovoid shallow substratum (MSS) traps were set. The myrmecofauna was additionally sampled by hand collection, leaf litter sifting, and sweep netting. Only five species were reported in the literature for the region of Mesta Valley prior to our study. Twenty species were recorded in total during the survey in Mesta Valley. Fifteen species are new to the area, and fourteen are thermophilic species. The collected specimens were preserved in 70-80% ethanol and deposited in the Institute of Biodiversity and Ecosystem Research collection at the Bulgarian Academy of Sciences (IBER). Additional studies will clarify the rates of penetration of thermophilic species after qualitative and quantitative comparison of the composition of the myrmecofauna, along the south–north gradients. This study is a part of the Project “Monitoring the effects of global climate changes through qualitative and quantitative analysis of model animal groups in selected corridors of thermophilic fauna penetration in Bulgaria” (funded by the National Science Fund of Bulgaria under grant contract number KP-06-N61/6 – 14.12.2022).