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The birch tree occupies the temperate climate region in North America and Europe. Every year between April and May, the birch trees pollinate intensively, which is a common cause of seasonal pollinosis which negatively affects the quality of life in many subjects. Pollen allergy (pollinosis) is among the leading allergic diseases. Sensitization to birch pollen is common in the European regions. Bet v 1 is the main birch pollen allergen which may induce specific IgE in 95% of patients sensitized with birch pollen. Bet v 1 belongs to the PR-10 class of proteins, which includes a large group of aeroallergens and common food allergens. Currently, according to the nomenclature of allergens, 27 variants (isoforms) of Bet v 1 proteins are discerned which may often differ in only several amino acids. The aim of this study was to determine the genetic diversity of the major birch pollen allergen Bet v 1 on the territory of the Republic of Belarus. We studied the pollen samples from birch collected in spring time. Recombinant vector constructs containing genes encoding different isoforms of Bet v 1 allergen were obtained. The nucleotide sequences of the cloned fragments have been determined. Three sequences were found to contain intronic regions interrupting the coding part of the gene. An abridged reading frame was detected in two other sequences. We evaluated the spectrum of Bet v 1 protein isoforms. The sequences obtained are related to 11 genetic variants of the studied allergen to a greater or lesser extent. The majority (86%) of the identified variants correspond to 7 isoforms of 2 isoallergens retrieved in the database of allergenic proteins, with Bet v 1.0101-like sequences being more common (42%), followed by the group with common Bet v 1.0104 variant (19%). The third position (11%) is occupied by Bet v 1.0102-like sequences. The Bet v 1.02 isoallergen is represented by a single variant which is closely similar to the Bet v 1.0204 isoform (3% of the total number of sequences under study). 7 Bet v 1 isoforms were identified within one phylogenetic tree. Bet v 1.0101 (Bet v 1a, X15877.1) was found to be the predominant isoform of the main birch pollen Bet v 1 allergen.