Using 108 strains of Vibrio parahaemolyticus isolated from seawater, we investigated ampicillin-resistance profiles and the genetic characterization of ${eta}$-lactamase (VPA0477). All of the strains studied, except one strain, were resistant to ampicillin. However, the strain that was susceptible to ampicillin had the same ${eta}$-lactamase gene as the ampicillin-resistant strains. We compared ${eta}$-lactamase promoter region sequences among five strains, including both ampicillin-resistant and -susceptible strains. In the susceptible strain, a nucleotide at position -19 in the methionine initiation codon for ${eta}$-lactamase was not present in the ampicillin-resistant strains. The genes in the region containing the gene VPA0477 were present in all of the tested strains, and LA-PCR analysis showed that the distance between VPA0474 and VPA0479 in all of the V. parahaemolyticus samples was precisely 5.7 kb. In V. parahaemolyticus ${eta}$-lactamase, four important structural features that are conserved in Class A ${eta}$-lactamases were present in the deduced amino acid sequences. Taken together, our study demonstrates that V. parahaemolyticus ${eta}$-lactamase is included in the Class A ${eta}$-lactamase group, and some nucleotides within the promoter region are of particular importance for ${eta}$-lactamase activity.