Two novel IMP-type metallo-β-lactamase variants, IMP-43 and IMP-44, were identified in multidrug-resistant Pseudomonas aeruginosa isolates obtained in medical settings in Japan. Analysis of their predicted amino acid sequences revealed that IMP-43 had an amino acid substitution (Val67Phe) compared with IMP-7 and that IMP-44 had two substitutions (Val67Phe and Phe87Ser) compared with IMP-11. The amino acid residue at position 67 is located at the end of a loop close to the active site, consisting of residues 60 to 66 in IMP-1, and the amino acid residue at position 87 forms a hydrophobic patch close to the active site with other amino acids. An Escherichia coli strain expressing blaIMP-43 was more resistant to doripenem and meropenem but not to imipenem than one expressing blaIMP-7. An E. coli strain expressing blaIMP-44 was more resistant to doripenem, imipenem and meropenem than one expressing blaIMP-11. IMP-43 had more efficient catalytic activities against all three carbapenems than IMP-7, indicating that the Val67Phe substitution contributed to increased catalytic activities against carbapenems. IMP-44 had more efficient catalytic activities against all carbapenems tested than IMP-11, as well as increased activities compared with IMP-43, indicating that both the Val67Phe and Phe87Ser substitutions contributed to increased catalytic activities against carbapenems.