Field inversion gel electrophoresis (FIGE) and contour-clamped homogeneous field (CHEF) electrophoresis were used to analyse the chromosome of Yersinia ruckeri. The 8 base-pair recognition endonucleases, NotI and SfiI, generated less than 47 DNA fragments whose size and distribution were appropriate for pulsed field separation. Each isolate displayed a characteristic restriction pattern, with about 20% of bands in common. Depending on the strain used, the estimated genome size for this bacterial fish pathogen ranged from 4460 to 4770 kilobase pairs.