The natural susceptibility of 74 Plesiomonas shigelloides strains isolated from humans (n = 50), water (n = 22) and animals (n = 2) to 71 antibiotics was examined. MICs were performed using a microdilution procedure with Mueller-Hinton broth and an inoculum of 1 x 10(6) cfu/mL. Plesiomonas strains were naturally susceptible or naturally susceptible and intermediate to tetracyclines, several aminoglycosides, aminopenicillins in combination with beta-lactamase inhibitors, all cephalosporins except cefoperazone, ceftazidime and cefepime, carbapenems, aztreonam, quinolones, trimethoprim, sulfamethoxazole, azithromycin, chloramphenicol, nitrofurantoin and fosfomycin. Uniform natural resistance was found to all penicillins tested, roxithromycin, clarithromycin, lincosamides, streptogramins, glycopeptides and fusidic acid. Plesiomonas strains were naturally resistant and intermediate to streptomycin, erythromycin and rifampicin. There were two susceptibility patterns to piperacillin/tazobactam, several cephalosporins and aztreonam. In contrast to a previous study with beta-lactam antibiotics, susceptibility testing of non-beta-lactams revealed no alterations of the MICs of most antibiotics, using different inocula and media. A database is described of the natural susceptibility of P. shigelloides strains to a wide range of antibiotics. It can be used for the validation of susceptibility test results of these bacteria.