A thrombin-like serine protease, jararassin-I, was isolated from the venom of Bothrops jararaca. The protein was obtained in high yield and purity by a single chromatographic step using the affinity resin Benzamidine-Sepharose CL-6B. SDS-PAGE and dynamic light scattering analyses indicated that the molecular mass of the enzyme was about 30 kD. The enzyme possessed fibrinogenolytic and coagulant activities. The jararassin-I degraded the Bβ chain of fibrinogen while the Aα chain and γ chain were unchanged.Proteases inhibitors, PMSF and benzamidine inhibited the coagulant activity. These results showed jararassinI is a serine protease similar to coagulating thrombin-like snake venom proteases, but it specifically cleaves Bβ chain of bovine fibrinogen. Single crystals of enzyme were obtained (0.2 mm×0.2 mm×0.2 mm) and used for X-ray diffraction experiments.