We examined the use of previously observed restriction fragment length polymorphisms (RFLPs) of a polymerase chain reaction (PCR)-amplified fragment of plasmid pEA29 for differentiating strains of Erwinia amylovora. The PCR fragment from E. amylovora strain CA11 contains a region of 8-bp tandem repeats which is predicted to cause the RFLPs. Examination of a collection of 93 strains revealed the repeat sequence GATTACA(GAATTACA)nGAATTACA in pEA29 with n ranging from 3 to 14. Selected strains were examined after growth in liquid culture to establish the stability of this character. Four strains originally with n = 14, 13, 7, and 3 repeats were grown overnight in liquid culture and streaked onto agar plates to produce individual colonies. Respectively, 4, 10, 1, and 0 out of 17 colonies per strain had an altered copy number when retested. Considering the instability in the number of repeats, it is concluded that the polymorphism in this region of pEA29 is not useful as a marker for following the migration of E. amylovora.