To increase the efficiency of breeding fiber and oil flax plants resistant to the pathogen, the sufficient set of the parent plants possessing effective R-genes is necessary. <...> Development of monogene-based resistant varieties will allow to reduce the time required for breeding, and provide essential suppression of disease. <...> In the present research we tested the resistance of world Linum usitatissimum L. gene pool specimens to highly aggressive Fusarium oxysporum f. lini monoisolates, and also studied haw the expressivity of R-genes was influenced by the temperature. <...> The study was conducted in 2006-2010 using 28 collection flax specimens characterized by high resistance to F. oxysporum f. lini population. <...> To rank the specimens on fusarium wilt resistance, the tests were conducted in the nurseries and a climatic chamber at optimal temperature (26-28 С), and the plant damage was estimated at early yellow ripeness or early development, respectively. <...> Hybridological analysis data were in line with phytopathological tests specifying genetic distinctions of specimens ¹ 3896, l. 6 (Russia); Siciliana 285, l. 4 (Italy) and Honkej 21, l. 4 (China) with effective resistance genes Fu 4, Fu 7 and Fu 8, respectively. <...> It was shown that at 26-28 C during the seedlings—«herringbone» period an expressivity of resistance genes can decrease owing to increased aggressiveness of some races of the pathogen. <...> At that, the effectiveness of Fu 7 gene was significantly influenced by the raised temperatures, whereas the effect of R-genes in the k-5657 (Minnesota, the USA) did not depend on the temperature. <...> The found sources possessing various Rgenes against fusarium wilt, when used in breeding, will help to avoid epiphytoty and to provide a sustainable flax production. <...> Keywords: fibre-flax, linseed, resistance, fusarial wilt, genetic sources, the temperature factor Flax (Linum usitatissimum L.) species, due to high adaptiveness, are widely cultivated in Russia. <...> These are fiber flax (Linum usitatissimum L. f. elongate) in northern and oilseed flax (Linum usitatissimum L. var. intermedia) in central and southern Russia [1]. <...> Many of today's domestic and foreign flax varieties are characterized by high and moderate resistance to the disease [5-7]. <...> Since transmission of the infection occurs primarily through the soil <...>