The seminal plasma is a very complex fluid， which surrounds sperm in semen. It contains numerous proteins including proteases and protease inhibitors that regulate proteolytic processes associated with protein activation and degradation. We previously identified a seminal protein， chicken liver trypsin inhibitor 1 (ClTI-1) over expressed in semen of roosters with high fertility， suggesting a role in male fertility. In the present study， we showed that ClTI-1 gene is actually SPINK2. Using normal healthy adult roosters， we showed that SPINK2 amount in seminal plasma was positively correlated with male fertility in chicken lines with highly contrasted genetic backgrounds (broiler and layer lines). Using affinity chromatography combined to mass spectrometry analysis and kinetic assays， we demonstrated for the first time that two chicken acrosin isoforms (acrosin and acrosin-like proteins) are the physiological serine protease targets of SPINK2 inhibitor. SPINK2 transcript was overexpressed all along the male tract， and the protein was present in the lumen as expected for secreted proteins. Altogether， these data emphasize the role of seminal SPINK2 Kazal-type inhibitor as an important actor of fertility in birds through its inhibitory action on acrosin isoforms proteins.