Mx (myxovirus-resistant) proteins are large interferon-induced proteins, which have a key role in innate immune response against viral infection in many vertebrates. Although, in mallards (Anas platyrynchos), one of the major natural reservoirs for influenza A virus, the role of Mx gene function in clearance of viral pathogens is still unclear. The avian Mx gene has an approximately 100 amino acids extension in the N-terminal called the avian specific region which is suggested to be under high levels of positive selection. In this study, the avian specific region for 96 wild mallards caught at Ottenby Bird Observatory on Öland, Sweden, was explored for diversity and signatures of selection. Sequencing and selection analysis showed relatively low total nucleotide diversity, a dN/dS ratio below one, and a negative value of Tajima’s D was found which indicates an excess of low frequency mutations, a sign of purifying selection. However, three codons in the nucleotide alignment were found to be positively selected, although, there is evidence for purifying selection overall. This could suggest that avian specific region is an important part of the protein, and due to this it is essential to conserve amino acid sequence to retain functionality. Further studies on the Mx gene are still necessary to gain more clarity in the role of Mx gene in Mallard and its evolutionary forces.