Escherichia coli, Salmonella species, and Pasteurella multocida are the major bacterial pathogens isolated from poultry. Difference in susceptibility to antibiotics by microorganisms has become a major factor in drug choice and success of treatment. Great concerns have been raised regarding emerging antimicrobial resistance among bacteria that may result in unpredictable antimicrobial susceptibility and failure of therapy. The primary objective of the present study was to determine the levels of antimicrobial susceptibility/resistance of E. coli, Salmonella species, and P. multocida isolated from diseased chickens. A total of 445 E. coli isolates, 387 Salmonella spp. isolates, and 80 P. multocida isolates from diseased chickens during the period ranging from 2001 to 2003 were obtained. Minimal inhibitory concentrations of 14 antimicrobial agents against each bacterial isolate were determined using a microbroth dilution assay described by the Clinical Laboratory Standards Institute. Resistance of E. coli isolates measured as follows: 98.20% were resistant to tilmicosin, 79.33% to tetracycline, 51.46% to spectinomycin, 44.04% to gentamicin, and 40% to ampicillin. Resistance to tetracycline was found in 72.61% of Salmonella spp. isolated, followed by resistance to spectinomycin (68.48%), ampicillin (63.57%), gentamicin (63.31%), and ticarcillin (61.76%). The resistance rate of P. multocida isolates to all antimicrobials tested was less than 5%, except for tetracycline (6.25%). In summary, E. coli and Salmonella isolates were sensitive to ceftiofur and fluoroquinolones but were resistant to other antimicrobials tested, while P. multocida isolates remained sensitive to all the antimicrobial agents tested in a 3-yr analysis.