Many bacterial secondary metabolites, including pharmacologically promising compounds, are synthesized by polyketide synthases (PKS) enzyme complexes. In the present work, nucleotide sequences of the genes encoding 16S rRNA and PKS of heterotrophic bacterial strains isolated from epilithic biofilms of the littoral zone of Lake Baikal were determined. On the basis of molecular phylogenetic analysis of the 16S rRNA genes, six heterotrophic strains were identified: Serratia fonticola 1А and 10А, Pseudomonas umsongensis K10-2 and K10-3, Rheinheimera tilapiae K18, and Flavobacterium sp. 43-09. Sequencing of cloned amplification products for PKS gene cluster revealed 33 sequences. Genes involved in biosynthesis of antibiotics (difficidine, erythromycin, curacin, mixalamide, corallopyronin, and myxothiazol) and cytostatics (romidepsin, spiruchostatin, and disorazol) were found among homologous sequences. The low homology (50-83%) of the PKS amino acid sequences of Baikal bacteria with sequences from GenBank attests to the potential capability of strains to produce new, not yet studied bioactive compounds. The obtained results show that the studied strains may be of practical interest for biotechnological application. [ABSTRACT FROM AUTHOR]