The most widely used technique for determination of fibrinogen concentration is the Clauss fibrinogen (FIB(Clauss)) assay, which measures the clotting time of plasma after addition of excess thrombin. More recently, the PT-derived fibrinogen (FIB(PT)) assay has been developed, based on the relationship between fibrinogen concentration and the kinetics of clot formation during the prothrombin time. The objective of this study was to compare the fibrinogen concentration determined by the FIB(Clauss) and FIB(PT) assays in citrated plasma samples from healthy dogs (n = 40), monkeys (n = 40), rabbits (n = 26), and rats (n = 58) by using an automated coagulation analyzer. Results of a t test analysis indicated that the mean plasma fibrinogen concentrations measured by the 2 assays for all 4 species were significantly different. According to Pearson correlation coefficients, the FIB(PT) assay displayed a high correlation (0.93 to 0.98) with the FIB(Clauss) assay for all species. When the FIB(PT) and FIB(Clauss) assays were compared by using Deming regression, positive or negative constant and proportional biases emerged for all species. Intra- and interassay coefficients of variation for the FIB(PT) and FI(BClauss) assays were 0.8% to 2.3% and 1.8% to 7.4%, respectively. In conclusion, the FIB(PT) assay is a rapid and economical method for estimating fibrinogen concentration in plasma samples from dogs, monkeys, rabbits, and rats. However, it should not be used without restriction. Further studies are required to investigate the performance of this assay in animals with various pathologic states, including coagulopathy, dysfibrinogenemia, and hypo- or hyperfibrinogenemia.