ISR1 is a small transposable element, identified in Rhizobium class IV strains by its high frequent mutagenic insertion into plasmid RP4. Hybridization studies showed that ISR1 is present in, multiple copies in Rhizobium class IV strains. Nucleotide sequence analysis revealed that ISR1 has a length of 1260 bp and is characterized by perfect inverted repeats of 13 nucleotides followed by a stretch of 28/29 nucleotides with imperfect homology. The insertion under study generated a target site duplication of 4 bp. ISR1 carries a large open reading frame, encoding a putative polypeptide of 278 amino acids (ORFA*), and three smaller ones in antiparallel direction (ORFs A1, A2, A3). Two of them are completely covered by the large open reading frame. No significant homology to 17 other known insertion sequence elements could be detected, either at nucleotide or at amino acid levels.