In this study, we demonstrated that the methyltransferase activity associated with Dam was essential for attenuation of Aeromonas hydrophila virulence. We mutated aspartic acid and tyrosine residues to alanine within the conserved DPPY catalytic motif of Dam and transformed the pBAD/damD/A, pBAD/damY/A, and pBAD/damAhSSU (with the native dam gene) recombinant plasmids into the Escherichia coli GM33 (dam-deficient) strain. Genomic DNA (gDNA) isolated from either of the E. coli GM33 strains harboring the pBAD vector with the mutated dam gene was resistant to DpnI digestion and sensitive to DpnII restriction endonuclease cutting. These findings were contrary to those with the gDNA of E. coli GM33 strain containing the pBAD/damAhSSU plasmid, indicating nonmethylation of E. coli gDNA with mutated Dam. Overproduction of mutated Dam in A. hydrophila resulted in bacterial motility, hemolytic and cytotoxic activities associated with the cytotoxic enterotoxin (Act), and protease activity similar to that of the wild-type (WT) bacterium, which harbored the pBAD vector and served as a control strain. On the contrary, overproduction of native Dam resulted in decreased bacterial motility, increased Act-associated biological effects, and increased protease activity. Lactone production, an indicator of quorum sensing, was increased when the native dam gene was overexpressed, with its levels returning to that of the control strain when the dam gene was mutated. These effects of Dam appeared to be mediated through a regulatory glucose-inhibited division A protein. Infection of mice with the mutated Dam-overproducing strains resulted in mortality rates similar to those for the control strain, with 100% of the animals dying within 2 to 3 days with two 50% lethal doses (LD50s) of the WT bacterium. Importantly, immunization of mice with a native-Dam-overproducing strain at the same LD50 did not result in any lethality and provided protection to animals after subsequent challenge with a lethal dose of the control strain.