The majority of Aeromonas spp. are innately resistant to ampicillin, therefore the aim of this thesis was to examine the resistance mechanism of A.salmonicida subspp. achromogenes and A.hydrophila to β-lactam antibiotics. Previous researchers had revealed the presence of two β-lactamases in the motile Aeromonas spp., a penicillinase with carbapenemase activity and a cephalosporinase. However, it was demonstrated in this thesis, that these β-lactamases had not been purified completely and that three β-lactamases could be purified from A.salmonicida subspp. achromogenes and A.hydrophila. Anion and cation exchange chromatography were employed to separate the β-lactamases of the strains which had been induced with sub-MIC doses of ampicillin, cefoxitin or imipenem. Substrate profiles of these purified β-lactamases revealed the presence of a cephalosporinase, a penicillinase and a highly unusual carbapenemase which cannot be detected with nitrocephin. Inhibitor profiles showed that the cephalosporinase and the penicillinase were serine-based, whereas the carbapenemase was a metallo-enzyme but was unusually sensitive to zinc and may be a new class of metallo-β-lactamase. The cephalosporinase is probably a class C β-lactamase whereas the penicillinase is probably a class A β-lactamase. An ampicillin-sensitive species, A.salmonicida subspp. salmonicida was also examined and found to contain a carbapenemase and a cephalosporinase, but no penicillinase, which will explain its sensitivity to ampicillin. The expression of these two β-lactamases appears to be co-regulated as the cephalosporinase was only expressed in the uninduced strain and the carbapenemase activity was much higher post-induction. During this PhD., the first quinolone-resistant clinical isolate of A.hydrophila was found. The gene encoding the α subunit of the DNA gyrase was therefore amplified and sequenced and compared with the sequence of a sensitive strain. A mutation was found in the codon of amino acid 83 which affected the ability of the quinolone drugs to bind to the DNA gyrase. Five laboratory mutants of A.hydrophila were also selected with either nalidixic acid or ciprofloxacin and the changes in the sequences of the gyrA genes were also determined to occur at amino acid 83 and/or 87.