Fine ultrastructure ofpolyvalent bacteriophages TT10-27 and KEY, isolated from affected with fire blight disease plant tissues, was studied using electron microscopy. Phages have isometric heads connected to short complex tail Key words: bacteriophages, Erwinia amylovora, morphology and structural organization o f virion.Morphological organization and ultrastructure of a phage particle is common criterion of bacteriophage classification and taxonomy [2]. According to Ackermann [2] phages of the order Caudovirales are subdivided into three families Myoviridae, Siphoviridae and Podoviridae due to their tail structure, and into subtypes according to head length. Tykhonenko [25] proposed phage classification based on the base plate shape or tail distal end if a base plate is absent. This approach allows the taxa of lower rank than family to be distinguished. Currently such classic criteria are not taken into consideration, however genomic and post-genomic studies are impossible without characterization of morphology of the object.Bacteriophages specific to Erwinia amylovora and other phytopathogenic and plantassociated bacteria were isolated from fire blight disease affected tissues of trees earlier [23]. Phages of two morphotypes: B1 (Siphoviridae) and C1 (Podoviridae) were observed among them [24]. Here we report results of electron microscopic studies of two representatives of the groups: phages TT10-27 and KEY Materials and methods.Bacteriophage TT10-27 was obtained when an isolate pMG (extracted from affected pear sample) was propagated on Erwinia "horticola" 60-1n and 4311 laboratory strains; phage KEY was obtained during reproduction of isolate pMA1 (extracted from affected quince) on E. amylovora K8(ATCC 29850) [23].Bacteriophages were obtained by fused lysis procedure [1] or by delayed lysis method de scribed in [24]. Phage concentration procedure included lysates clarification by centrifugation for 1h at 5000g, 10°C, with next phage particles sedimentation in rotor SW 28 Spinco L7-70 at 26000 rpm for 2 hours at 10°C. Obtained precipitate was re-suspended in STM buffer [18] and purified from cell debris and particles' conglomerates on microcentrifuge ELMI at 11000 rpm for 10 min.More thorough purification of virions was achieved using CsCl stepwise density-gradient centrifugation (1,42 g/cm3 i 1,60g/cm3). CsCl solution (5M) was prepared in buffer STM and in water. The latter contributed to the phage virions' destruction. Phage suspension of 0.5-1ml volume was applied on the gradient and centrifuged for 3 hours in SW 55 rotor, at 30 000 rpm, 10 °C.Dialysis of CsCl-purified virus was carried out against a 1000-fold excess of ST buffer [18] for 24 hours at 4 °C.