The antimicrobial resistance of Pseudomonas aeruginosa to beta-lactams, aminoglycosides and ciprofloxacin was investigated in two Latin American hospitals, one in Venezuela and the other in Paraguay. The resistance of P. aeruginosa was investigated in 1,481 clinically isolated strains, 988 from Asunción and 493 from Caracas, collected between 1996 and 1999. Susceptibility was assessed by the disk diffusion method according to the National Committee for Clinical Laboratory Standards. 11.4% were resistant to cefoperazone, 11.4% to ceftazidime, 12.8% to piperacillin, 13.6% to amikacin, 18.2% to gentamicin, 11.1% to ciprofloxacin, and 6.7% to imipenem. There were significant differences in resistance patterns between isolates from Asunción and Caracas. Resistance was higher in Caracas. Despite similar antibiotic usage policies and other measures, differences in the resistance patterns of P. aeruginosa are evident in this study. The clinical and therapeutic implications of this resistance suggest the need to maintain surveillance in local settings, especially in developing countries such as Venezuela and Paraguay.