In this thesis, the anti-Acinetobacter activity of the newer fluoroquinolones are investigated by MIC and kill-curve and it is shown that they are 2-16-fold more potent than ciprofloxacin. The target enzymes GyrA and ParC are investigated by polymerase chain reaction, restriction fragment length polymorphism and sequencing. It was found that a serine 83 mutation in GyrA conferred resistance to ciprofloxacin and also to the newer fluoroquinolones except trovafloxacin. Strains exhibiting high-level resistance was found to carry the GyrA mutation in addition to a serine 80 mutation in ParC. The selection of fluoroquinolone resistance in vitro was investigated by step-wise selection in order to investigate the mutational frequency and the development of resistance. It was found that ciprofloxacin selects for a GyrA mutation in one-step at a high frequency. The newer fluoroquinolones however required multiple steps of low frequency selection to acquire this mutation. ParC mutations were found not to be required for high-level resistance suggesting an alternative resistance mechanism. Mutants were subcultured on antibiotic-free media and their fluoroquinolone MIC’s were re-tested to look for spontaneous revertants. A revertant was investigated by sequencing of gyrA and parC but no changes were found. Narrow-range MIC’s with non-fluoroquinolones showed small differences in MIC between parent, mutant and revertant suggesting changes in permeability. The outer membrane profile was investigated and a porin of 45.5kDa was found to have increased expression in the mutant and this level was lower in the revertant. The porin was extracted and N-terminally sequenced. It was found to have homology to porin D in Pseudomonas aeruginosa, a porin associated with imipenem resistance. The narrow-range MIC’s of imipenem followed this pattern.