In addition to von Willebrand factor and fibrinogen, plasma fibronectin (pFN) contributes significantly to thrombus development in arteries. Complete deficiency of plasma fibronectin (in pFN conditional knockout mice) affects thrombus formation and growth and subsequent occlusion of injured arterioles in vivo. We wanted to study a more physiological decrease of plasma fibronectin without having to induce interferon production in the mice to excise the FN gene. Reduced pFN levels are common in patients with liver disease, sepsis and following trauma or surgery. To evaluate the effect of a reduced amount of plasma fibronectin on thrombus formation at arterial shear rate, we used the ferric chloride model of arterial injury in fibronectin heterozygote mice which have 50% of the normal plasma level of FN. Two groups of mice were observed by intravital microscopy - FN heterozygote (FN+/−) and corresponding age matched wild-type controls (FN +/+). The reduced level of pFN did not affect the early platelet interaction with subendothelium but caused a delay of several minutes in appearance of the first thrombus in the injured arteriole (15 min in FN +/− vs 6 min in FN+/+, p<0.001). Although the thrombi were stably anchored to the vessel wall, the growth of the thrombus was slowed down because of the constant platelet shedding in fibronectin heterozygote mice. Consequently, this lack of firm adhesion of the platelets led to occlusion delay (36 min in FN +/− vs 28 min in FN+/+, p<0.01) with the majority of arterioles in the FN+/− mice not occluding by 40 minutes after injury. In conlusion, the phenotype of thrombosis in the mice with 50% of normal pFN level strikingly resembles the phenotype observed in mice with induced complete deficiency of pFN. This further emphasizes the fundamental role of this plasma protein in thrombosis in vivo in the arterial system.