Experimental and in silico PCR analysis targeting ISAba11 and TnAbaR islands in 196 epidemiologically unrelated Acinetobacter strains representative of ≥19 species were performed. The first two Acinetobacter baumannii ISAba11 elements identified had been found to map to the same site on TnAbaR transposons. However, no further evidence of physical linkage between the two elements was demonstrated. Indeed, examination of 25 definite or putative insertion sites suggested limited sequence specificity. Importantly, an aacC1-tagged version of ISAba11 was shown to actively transpose in A. baumannii. Similarity searches identified nine iso-ISAba11 elements in Acinetobacter and one in Enhydrobacter and single representatives of four distant homologs in bacteria belonging to the phyla "Cyanobacteria" and Proteobacteria. Phylogenetic, sequence, and structural analyses of ISAba11 and/or its associated transposase (Tnp(ISAba11)) suggested that these elements be assigned to a new family. All five homologs encode transposases with a shared extended signature comprising 16 invariant residues within the N2, N3, and C1 regions, four of which constituted the cardinal ISAba11 family HHEK motif that is substituted for the YREK DNA binding motif conserved in the IS4 family. Additionally, ISAba11 family members were associated with either no flanking direct repeat (DR) or an ISAba11-typical 5-bp DR and possessed variable-length terminal inverted repeats that exhibited extensive intrafamily sequence identity. Given the limited pairwise identity among Tnp(ISAba11) homologs and the observed restricted distribution of ISAba11, we propose that substantial gaps persist in the evolutionary record of ISAba11 and that this element represents a recent though potentially highly significant entrant into the A. baumannii gene pool.