A database is described of the natural susceptibilities of 70 Rahnella strains to 71 antibiotics. MIC values were determined by a microdilution procedure and evaluated by a table calculation program. Rahnella aquatilis and R. aquatilis-related strains were naturally resistant to amoxycillin, ticarcillin, fosfomycin and to antibiotics to which other species of Enterobacteriaceae are also intrinsically resistant, i.e. macrolides (except azithromycin), benzylpenicillin, oxacillin, rifampicin, fusidic acid, lincosamides and glycopeptides. Rahnella strains were also naturally resistant or intermediate to cefazolin, cefuroxime and loracarbef. All rahnellae were naturally sensitive or intermediate to doxycycline, minocycline, aminoglycosides, some penicillins and cephalosporins, carbapenems, aztreonam, quinolones, sulfamethoxazole, trimethoprim, cotrimoxazole, chloramphenicol and nitrofurantoin. Bimodal or broad MIC distributions were seen for several antibiotics, e.g. quinolones and cephalosporins. With the exception of quinolones no differences in natural antibiotic susceptibility were seen between reference strains of Rahnella genomovar 1 (n=6) and 2 (n=7). Reference strains of genomovar 1 were pyrase-positive and more susceptible to quinolones than reference strains of genomovar 2, which were pyrase-negative. By discrimination of all rahnellae in the pyrase-positive and pyrase-negative strains the MIC distributions for quinolones became smaller and unimodal. Under the conditions described pyrase might be a parameter to differentiate strains of Rahnella genomovars 1 and 2.