Linseed varieties with different content of α-linolenic acid (LIN) and a different ratio of fatty acids are required for various industries. Vrinten et al. (2005) revealed that both fad3a and fad3b genes carried point mutations leading to premature stop codons in the line 593-708, resulting in 2–3% of the LIN content. It is possible to accelerate the breeding process by developing of effective molecular markers allowing to identify the mutant alleles of fad3A and fad3B genes, which makes it possible to select linseed in two directions - high and low LIN content, significantly improving the efficiency of the breeding programs. To identify the mutant allele of fad3a gene, primers were developed by Vrinten et al. (2005) were used. To identify the mutant allele fad3b gene, we sequenced fragments of the fad3b gene of low-linolenic linseed variety Amon and high-linolenic variety Spartak. Comparison of the wild-type and mutant sequences identified the point mutation in the fad3b gene resulted in the loss of a BalI site. Based on the nucleotide sequence data, we developed the CAPS marker MutFad3 (Lemesh et al., 2017). Primers were designed in such a way that the amplification product (600 bp) would contain the mutation, and digestion of the product with the BalI endonuclease would produce fragments with distinguishable sizes (385 and 215 bp). Low-linolenic plants carry markers for both mutant genes, and high-linolenic plants carry markers for both wild-type genes. Plants carrying a homozygous mutation at a single fad3 locus had intermediate levels of LIN. In the Institute of Flax of the National Academy of Sciences of Belarus, the linseed variety "Dar" was created based on the results of molecular-genetic and breeding studies. The variety is medium-ripening, blue-flowered. Seeds are brown, large. The content of αlinolenic acid is about 40.1%. The oil of this variety is suitable for use in medicine and perfumery.