Aims: Knowledge on factors of Aeromonas caviae promoting the formation of biofilms on surfaces. Methods and Results: In nutrient broth under agitation, A. caviae LMG 13455 was able to form biofilms on the surfaces of glass flasks, but such biofilm formation was inconsistent. A derivative of this strain, called M12, promoted the rapid formation of reliable and strongly adherent biofilms with about half of the cells being adhered. In contrast with its parent, M12 was hydrophobic, displayed auto-aggregation in liquid medium, synthesized very little polysaccharides and was defective in swimming and swarming motilities, together with the appearence of a characteristic phenotype on motility soft agar. Further analyses demonstrated that most of these properties were related to a hyperpiliation of the cells through the presence of type IV pili, and suggested that a mechanism of genetic variation, by altering the nature of motility appendages, allows the variant bacteria to attach on inert surfaces. Conclusions: M12 is a stabilized variant of the parental strain, promoting strongly adherent biofilms through the type IV pili. Significance and Impact of the Study: The wild-type strain A. caviae LMG 13455 include subpopulations that are likely implicated in its adaptation to different environmental changes.