Objective: To study the genetic diversity of Culex theileri flavivirus and the spread of this virus among Spain, Portugal and Turkey. Methods: A database consisting of 55 sequences of the NS5/3'UTR region of Culex theileri flavivirus group downloaded from GenBank were aligned and manual edited with Bioedit. ModelTest v. 3.7 was used to select the simplest evolutionary model that adequately fitted the sequence data. Maximum likelihood analysis was performed using MEGA7. The phylogenetic signal of the dataset was investigated by the likelihood mapping analysis. Results: The phylogenetic tree showed three clusters. Myanmar sequences clusterd together with Turkish sequences, Spain and Portugal strains grouped together and two Turkish sequences grouped separately. Selective pressure analysis showed a moderate percentage of sites (22.5%) under pervasive negative selection and only 1% under pervasive positive selection. The sites subject to selective pressure in CTFV RdRp NS5 fragments have been located onto the predicted three-dimensional structure. Conclusions: Phylogenetic and evolutionary analysis can be an important tool for understanding the evolutionary impact of the probable contemporary existence between non-pathogenic and pathogenic flaviviruses among these vectors.