Three segregating F-2 populations were developed by self-pollinating 3 black rot resistant F-1 plants, derived from a cross between black rot resistant parent line 11B-1-12 and the susceptible cauliflower cultivar 'Snow Ball'. Plants were wound inoculated using 4 isolates of Xanthomonas campestris pv. campestris (Xcc) race 4, and disease severity ratings of F-2 plants from the three populations were scored. A total of 860 arbitrary oligonucleotide primers were used to amplify DNA from black rot resistant and susceptible F-2 plants and bulks. Eight RAPD markers amplified fragments associated with completely disease free plants following black rot inoculation, which segregated in frequencies far lower than expected. Segregation of markers with black rot resistance indicates that a single, dominant major gene controls black rot resistance in these plants. Stability of this black rot resistance gene in populations derived from 11B-1-12 may complicate introgression into B. oleracea genotypes for hybrid production.