Acinetobacter baumannii is an important nosocomial pathogen, usually found on various surfaces in the hospital environment. Biofilm formation is a virulence factor for A. baumannii. In this study, 17 blood isolates of A. baumannii were studied for the ability to form biyofilm on the surface of polystyrene, and ultrastructural cell wall properties of biofilm-formatted strains were evaluated. The ability of Acinetobacter strains to form biofilm was determined on polystyrene microtiter plates using brain heart infusion broth supplemented with 0.25% glucose as the growth medium. Additionally, transmission electron microscopy of strains was performed for the presence of flagella, pili, and an amorphous material covering the cell described for some other bacteria. Biofilm formation was detected in 9 (52.9%) of A. baumannii strains. Ultrastructural analysis showed that an amorphous exopolysaccharide was observed around the biofilm-positive strains while flagella or pili were not detected in any of them. The results suggest that exopolysaccharides are the structures that may play the major role in the biofilm formation of A. baumannii.