Twenty-seven proteolytic Acinetobacter strains differing phenotypically from the 12 previously described Acinetobacter species were studied by DNA/DNA hybridization using the S1 nuclease method to assess their relatedness. Five DNA groups (genomic species 13 to 17) containing 20 strains were delineated. Seven strains remained ungrouped. Within species, the level of DNA relatedness to the reference strains ranged from 64 to 99%, with delta Tm values below 3.5 degrees C. DNA group 13 was 31 to 42% related to group 14. DNA group 15 was 59 to 69% related to group 16, with delta Tm values between 4.5 and 6 degrees C. DNA group 17 was 51 to 61% related to DNA groups 15 and 16 with delta Tm values between 5.5 and 7.5 degrees C. The seven ungrouped strains were 28 to 60% related to the five newly delineated genomic species with delta Tm between 6.5 and 13.5 degrees C. Reference strains of the five genomic species were 5 to 22% related to the type or reference strains of the 12 Acinetobacter genomic species previously described. Biochemically, DNA groups 13 to 17 and ungrouped strains could not be separated unambiguously and therefore are not named.