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FIFTEEN bacterial isolates were isolated from soil samples collected from Matrouh Governorate, Egypt and screened for antagonistic activity against some bacterial and fungal plant pathogens. Isolate SM3 showed the highest inhibitory effect against the tested plant pathogens. This isolate was identified using 16S rRNA encoding gene sequence analysis as Bacillus atrophaeus (GenBank accession no.: MZ741525). Its antagonistic activity was confirmed against four soft rotting bacterial isolates namely: Erwinia carotovora subsp. carotovora (Ecc) isolates: Pep7C, Cab21B2 and Cab45B and Erwinia chrysanthemi (Echr) isolate Car1B as well as four fungal plant pathogens namely: Curvularia lunata, Fusarium oxysporum, Aspergillus awamori and Penicillium sp. The antagonistic effect of B. atrophaeus SM3 on the fungal pathogens was also investigated using the mycelial reduction test where the highest mycelial reduction percentage was recorded against C. lunata (93.4%) followed by Penicillium sp. (77.8%) then F. oxysporum (61.7%) and A. awamori (41.0%).Transmission Electron Microscopy examination of the fungal isolates antagonized by B. atrophaeus SM3 showed different morphological changes including cell shrinking, alterations in the cell wall thickness, light-scattering cytoplasm, and cytoplasmic reduction. Analysis of the crude extract of B. atrophaeus SM3 by GC-mass spectroscopy indicated the presence of many compounds such as Tridecanoic acid (TDA), Pyrrole, Pentadecanoic acid and different types of fatty acids, with known biological activity for controlling pathogenic microorganisms. These results indicate that B. atrophaeus SM3 could be used as an eco-friendly biocontrol agent for successful plant disease management strategies. [ABSTRACT FROM AUTHOR]