Biofilm formation of pathogen bacterium is currently one of the most widely studied topics; however, little is known regarding pathogen bacteria biofi lms in aquaculture. Aeromonas hydrophila is a representative species of the genus Aeromonas, which has been recognized as a common pathogen, is associated with many diseases in aquatic animals, and causes signifi cant mortality. The objectives of this study are i) to confi rm that A. hydrophila can form biofi lms on abiotic substrates and construct a biofi lm growth curve for this bacterium; ii) to identify the genes that play crucial roles in A. hydrophila biofi lm formation. The biofi lm growth curve of A. hydrophila was constructed using a crystal violet assay, which showed that biofi lm formation for this bacterium is a dynamic process. Next, a mutant library of pathogenic A. hydrophila B11 was constructed using the mini-Tn10 transposon mutagenesis system. A total of 861 mutants were screened, and 5 mutants were stably defi cient in biofi lm formation. Molecular analysis of the mutant B112 revealed that the open reading frame that encodes the protein MshQ was disrupted.