ABSTRACT Enteropathogenic Escherichia coli (EPEC) is a major of cause of diarrhea among children in developing countries. Although EPEC is a human specific pathogen, some related strains are natural pathogens of animals, including laboratory-bred rabbits. We have identified two chromosomal loci in rabbit-specific EPEC (REPEC) O15:H− strain 83/39, which are predicted to encode long polar fimbriae (LPF). lpf _R154 was identical to a fimbrial gene cluster, lpf _O113 , identified previously in enterohemorrhagic E. coli (EHEC) O113:H21. The second locus, lpf _R141 , comprised a novel sequence with five predicted open reading frames, lpfA to lpfE , that encoded long fine fimbriae in nonfimbriated E. coli ORN103. The predicted products of lpf _R141 shared identity with components of the lpfABCC′DE gene cluster from EHEC O157:H7, and the fimbriae were similar in morphology and length to LPF from EHEC O157:H7. Interruption of lpf _R141 resulted in significant attenuation of REPEC 83/39 for rabbits with respect to the early stages of colonization and severity of diarrhea. However, there was no significant difference in the number of bacteria shed at later time points or in overall body weight and mortality rate of rabbits infected with lpf _R141 mutant strains or wild-type REPEC 83/39. Although rabbits infected with the lpf _R141 mutants did not develop severe diarrhea, there was evidence of attaching and effacing histopathology, which was indistinguishable in morphology, location, and extent compared to rabbits infected with wild-type REPEC 83/39. The results suggested that lpf _R141 contributes to the early stages of REPEC-mediated disease and that this is important for the development of severe diarrhea in susceptible animals.