Abstract: The purpose of this research was to investigate the presence of secondary bacterial or yeast contamination of otitis externa in dogs and to analyze the antimicrobial susceptibility of those microorganisms. Clinical manifestations of external otitis were evaluated. Samples from the ear canal of 42 dogs with otitis externa were submitted to direct microscopic examination and cultured in blood agar and Sabouraud dextrose agar with chloramphenicol and cycloheximide. Otitis externa was more frequent in dogs between 2 and 5 year old 19 cases (55.9%) followed by up to 2 years 9 cases (26.5%). The most common dog breeds affected from otitis externa were crossbred (23.8%), German shepherd (16.6%) followed by cocker spaniel (14.3%) and Fox terrier (11.9%). Among 42 swab ear samples from dogs with otitis externa, microorganisms were isolated in 50% of cases as mono-infections (21 cases), involving mainly Staphylococcus pseudintermedius (10 cases), followed by Malassezia pachydermatis (8 cases), Streptococcus spp. (2 cases) and Staphylococcus aureus (one case) and other cases (13) as poly-infections (31%). The other microbial agents Proteus mirabilis in 4 cases, Pseudomonas aeruginosa in 5 cases and Staphylococcus aureus in 2 case were found also in poly-infection. All of the bacterial isolates tested were highly susceptible to amoxicillin-clavulanic acid, gentamycin, chloramphenicol and resistant to ampicillin, lincomycin and streptomycin.