To investigate the relationship between sensitivity varieties,mutations,efflux pump activities and crossresistance of selected strains after they were selected. The strains of Aeromonas hydrophila(Ah) from cultured fish sensitive to fluoroquinolone were selected in vitro stepwise exposure to increasing concentration of Norfloxacin (NF) and Enrofloxacin (EN) on solid medium, aiming to gain the fluoroquinolone-resistant isolates. Then genes of gryA and parC from selected strains were amplified and sequenced. The minimal inhibitory concentrations (MICs) of selected and non-selected antimicrobials as well as the MICs after efflux pump inhibition 1-Methyl-2- pyrrolidinone (NMP) was added to each strain were determined by broth microdilution method. Results shows the MICs of EN and NF increased 409.6-and 4096-fold after selection, respectively, while the MICs of non-selected fluoroquinolone and other antimicrobials also changed greatly. Mutations happened in the quinolone resistancedetermining region (QRDR) encoded by gyrA and parC genes after being selected: Ser83→Ile mutations of GyrA and Ser87→Ile/Arg mutations of ParC were identified. After NMP was added, MICs of all selected strains have declined in different degrees. Cross-resistance of selected strains is closely related to the strain, among the selected strains No. 3 and 8 have no cross-resistance to16 kinds of non-selected antibiotics, and the strains selected by EN and NF have nearly no cross-resistance to aminoglycosides, rifamycins and aminoglycosides(except GM), rifamycins, separately. All selected strains have great cross-resistance to tetracyclines and chloramphenicols. The resistant mechanisms of Ah to fluoroquinolone might include the target gene mutations and an active efflux system, and we should also pay more attention to the cross-resisance when choosing antimicrobials.