【Objective】 To investigate the effect of the sub-inhibitory concentration of quinolones on resistance to quinolones in Aeromonas hydrophila and its mechanisms. 【Method】 The quinolone sensitive strains of A.hydrophila from clinical and ATCC7966 were selected in vitro stepwise exposure to increasing concentration of nalidixic acid (NAL) and ciprofloxacin (CIP) on solid medium. The quinolone resistance-determining region (QRDR) was sequenced to determine the mutation for quinolone resistance. The minimal inhibitory concentrations (MICs) of selected and non-selected antimicrobials as well as MIC by efflux pump inhibition cyanide chlorophenyl hydrazone (CCCP) of each strain were determined by broth microdilution method. The relationship between sensitivity varieties, mutations and efflux pump activities were analyzed. 【Result and conclusion】 The MICs of NAL and CIP increased 1 024-and 64 000-fold after selection, respectively, while the MICs of other antimicrobials increased diversely. When MIC of NAL or CIP in A.hydrophila reached 16 μg/mL or 32 μg/mL, Asp87→Tyr or Ser83→Arg, mutations of GyrA were identified, but no mutations of ParC were found. After CCCP was added, only MICs of fluoroquinolones caused a slight decrease, implying that it might include the quinolone mechanisms of target gene mutations and an active efflux system in A.hydrophila.