Specific regions of the P1 adhesin structural gene of Mycoplasma pneumoniae hybridize to various parts of the mycoplasma genome, indicating their multiple-copy nature. In addition, restriction fragment length polymorphisms and sequence divergence have been observed in the P1 gene, permitting the classification of clinical isolates of M. pneumoniae into two groups, I and II. These data suggest that the observed P1 gene diversity may be explained by homologous recombination between similar but not identical multicopy P1-related sequences and the P1 structural gene. We used oligonucleotide probes specific to the diverged regions of the group I and group II P1 structural genes to clone and sequence multicopy P1-related DNA segments. We detected sequences in group I M. pneumoniae isolates that were homologous not only to the group I P1 structural gene but also to the diverged regions of the group II P1 structural gene. Likewise, sequences in group II clinical isolates that were homologous both to the group II P1 structural gene and the diverged regions of the group I P1 structural gene were detected.