The Diaemus youngi is a vampire bat (filo Chordata), found in American. This and two other species of bats (Demodus rotundus and Diphylla ecaudata) feed on vertebrates blood. In this work we have evaluated the action of the saliva from Diaemus youngi in the hemostatic system. Bat's saliva was collected after induction of salivation by pilocarpin from a specimen obtained from the East area of Brazil. The protein concentration was of 12.3 mg/ml determined by Bradfor's method and showed an eletrophoretic profile in 15% SDS-PAGE with several bands. Saliva (25 mg protein/assay) increased aPTT values (117.8s) about two times compared with control values (69.2s) and TT values from 24s to 33.9s. However, the PT did not show an alteration in the presence of saliva. The experiments realized with chromogenic substrates, S-2251 for plasmin and S-2288 for tissue plasminogen activator substrate showed that the saliva was active for both substrates. However, partial results showed insignificant values of plasminogen activation. Besides, the saliva increased the 7.2-fold fibrinoclotting time induced by thrombin. Finally, the incubation of fibrinogen or fibrin with 61.5 µg of saliva was carried for 3hs and were then analysed by SDS-PAGE. Our results show that the a-chain from fibrin and fibrinogen were degradated. In conclusion, our data suggests that the vampire bat Diaemus youngi saliva interfere in the hemostatic system in part due to a fibrinogenolytic activity.