MIC testing of Vibrio anguillarum isolates recovered from diseased farmed Atlantic cod revealed oxolinic acid MICs of < or =0.001, 0.06, and 16 microg ml(-1). Single gyrA Ser-Ile substitutions were identified at position 83 of the intermediate and resistant strains, while a parC Ser-Leu substitution at position 85 was found only in the resistant strain.