Cytochrome c oxidase encoded by multiple mitochondrial genes (COXI, COXII, and COXIII) and nuclear genes is an essential component of the mitochondrial respiratory chain that catalyzes the reduction of molecular oxygen by reduced cytochrome c. Subunits COXI and COXII of cytochrome c oxidase are known to play the most essential role in proton pumping and electron transfer. In this study we screened the somatic mitochondrial COXI gene of infertile men suffering from asthenospermia (n=34) in comparison to normozoospermic infertile men (n=32) and fertile men (n=100) from the Tunisian population. A novel homoplasmic missense mitochondrial mutation (m.6375A>G) was found in 5 asthenospermic patients (14%) but not in any of normozoospermic infertile men and fertile men. This mutation substituting the isoleucine at position 158 to valine in a highly conserved amino acid induces a reduction of the hydropathy index (from +1.920 to +0.239) and a decrease of the protein 3D structure number (from 50 to 26) as shown by PolyPhen bioinformatic program.