The taxonomic composition of bacteria associated with the proboscis worm Cephalothrix simula was studied and screening of the tetrodotoxin (TTX)-producing bacteria was carried out using confocal laser scanning microscopy and polyclonal antibodies. Bacterial isolates were identified using the 16S rRNA gene sequencing and phenotypic characteristics. A Bacillus species was found to be responsible for tetrodotoxin production in proboscis worm C. simula. Vibrio spp. dominated the associated microflora (68.18% of the total number of isolates). The analysis of sensitivity of 16 strains to antibiotics of various classes revealed multiple resistance to three or more antibiotics in all the studied isolates. Poor growth of most of the isolates on all the laboratory media was an indirect confirmation of the symbiotic relationships between the micro- and macroorganisms.