The prevalence of the cphA gene or related carbapenemase-encoding genes was investigated in 114 Aeromonas strains belonging to the six species of major clinical interest. A species-related distribution of cphA-related sequences was observed. Similar sequences were found in A. hydrophila, A. veronii bv. sobria, A. veronii bv. veronii, and A. jandaei, but not in A. caviae, A. trota, or A. schubertii. However, a single A. caviae strain (of 62 tested) was found carrying cphA-related sequences, suggesting the possibility of the horizontal transfer of this gene to species which normally do not carry it. Production of carbapenemase activity was detectable in 83% of the hybridization-positive strains but in none of the hybridization-negative ones. When it was present, carbapenemase activity was always inhibitable by EDTA. Either carbapenemase-producing or not, Aeromonas strains appeared to be susceptible to imipenem when in vitro susceptibility testing was performed with inocula of conventional size (10(5) CFU), for which MICs were always < or = 1 microgram/ml. With a larger inoculum (10(8) CFU), the MICs for carbapenemase-negative strains always remained < or = 1 microgram/ml, while those for carbapenemase-producing strains were always > or = 4 micrograms/ml, being usually higher than the breakpoint for susceptibility. The present results indicate that the production of metallocarbapenemase activity, apparently encoded by cphA homologs, is widespread among some of the Aeromonas species of clinical interest (A. hydrophila, A. veronii bv. sobria, A. veronii bv. veronii, and A. jandaei) and that imipenem MICs for carbapenemase-producing strains are subjected to a relevant inoculum size effect.