even different fungi were isolated from the necrotic root tissues of infected Flax seedlings or older plants. Samples used in isolation were randomly collected from the experimental flax plots at Giza Agricultural Research Station. Fusarium spp. were isolated most frequently comprising 41.09%, while frequencies of isolation of the other fungi ranged from 1.00 to 23.97%. Regression analysis revealed that root colonization incidence (RCI) and root colonization severity (RCS) and relationship of rootcolonizating fungi of flax conformed to the linear model. According to the generated model, RCI accounted for 89.8% of the total variation in RCS. A total of 52 randomly selected isolates from infected roots were tested for pathogencity on flax cultivar Sakha 1 under greenhouse conditions. The results of pathogenicity test demonstrated that Fusarium spp. are the major causal agents of flax seedling blight as they accounted for 54% of the pathogenic isolates in the test. A total of 103 monosporic Fusarium isolates were randomly colleced from eight governorates and identified to species level. F.oxysporum (63.60%) and F.solani (27.35%) were the most predominant species. Other species were F.moniliforme (3.88%), F.lateritium(1.04%), F.semitectum (0.66%) and unidentified Fusarium spp. (3.49%). RCI and RCS relationship of rootcolonizing fusaria of flax conformed to the linear model. According to the generated model, RCI accounted for 75.3% of the total variation in RCS. Of the 103 isolates, a random sample of 32 isolates were tested for pthogenicity on seedlings of flax cultivar Giza 10 under greenhouse conditions. The results of the pathogenicity test showed that 66.67% of pathogenic isolates belonged to F.oxysporum, while 33.33% belonged to F.solani. The high frequencies F.oxysporum and F.solani and their ability to cause considerable losses during seedling stage, strongly suggest that they are the most important fusaria involved in the etiology of seedling blight and root rot of flax in Egypt. Grouping the isolates of F.oxysporum, F.solani, and F.lateritium by cluster analysis, based on their virulence patterns was neither related to their geographic origins nor to species.